This is set a couple of months after 2x13. Zac (eventually) accepted the situation and bonded with his newfound sister. Those who know them best watch, and reflect, on the siblings' interactions.

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(ONDINA)

"That was totally not fair, you had a head start!" Zac proclaimed, splashing a bit of water towards Mimmi playfully.

Clearly unwilling to admit any wrongdoing, the brunette simply smiled. "I won the race fair and square." She shot back, easily dodging his next splash. "Right Ondina?"

The other mermaid opened her mouth to reply but, before she could proclaim that Mimmi had, in fact, won the race to Mako, Zac sent another small wave of water hurling in his sister's direction. This time, it hit its mark, and Mimmi was left sputtering in amused shock.

At first, Ondina found herself smiling at the good-natured exchange. The two had only recently been reunited and, she supposed, they had years of sibling bickering to catch up on.

As she watched the play-fight escalate, however, the blonde could not suppress a slight pang of jealously. The two were laughing and cavorting together, apparently no longer aware that a third figure was present. And Ondina could not stop herself from feeling like she had been replaced.

Until recently—until Zac—she and Mimmi had always acted like surrogate sisters. Neither of them (as far as they had known) had siblings of their own, and they had always been extremely close. Inseparable.

But once her best friend had discovered that the "land boy" they had come to thwart was, in fact, her long lost brother, things had begun to change.

She and Mimmi were still great friends, of course, but she had started to notice that Zac was the first one she turned to when she needed support or companionship. Ondina couldn't really blame her—she probably would have been the same if things had been reversed, and she knew that Mimmi was eager to get to know the only family member she had left.

But that didn't stop it from stinging sometimes.

"Ondina, want to race around the island with us?" Mimmi asked, her blue eyes shining with youthful glee. There was no doubt that she was enjoying her afternoon, and Ondina decided that it would be best not to bring down their mood.

"Thanks." She replied, forcing a lighthearted smile. "But I'm a bit tired. You two go on ahead without me, I'll see you later."

"Okay, if you're sure." Mimmi nodded before giving her friend a warm hug. "We'll see you later."

Zac nodded too, and offered her a cheery goodbye before turning back towards his sister.

"Last one back is a rotten codfish!" He proclaimed.

And with that, they were gone.

Ondina was alone.

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(EVIE)

"Are you sure she'll like it?" Zac asked nervously, gesturing the small gift bag on his hand.

Evie nodded, giving her boyfriend a reassuring pat on the back. "I'm 99.999% sure." She replied with a smile. "She's going to love it."

They were currently on their way to Rita's house where, very shortly, they were planning to celebrate Mimmi's birthday. It was the first festive occasion to arrive since they had all met and Evie knew that Zac was worried about letting his sister down.

It had taken a fair bit of time but, once Zac had finally allowed himself to accept the truth, he and Mimmi had begun to grow close. It was hard to deny that they shared a connection and Zac had (eventually) become eager to learn anything he could about his biological family.

The siblings had been spending a fair bit of time together during the past few months— and, as happy as she was to see Zac bond with his sister, Evie had to admit that it had taken a bit of adjustment on her part to accept the new status quo.

But it had not taken long to see why they both needed a bit of time to work things out and get to know each other. Mimmi had always felt alone, adrift without a family. Having lost her own mother at a young age, Evie could understand the mermaid's desire to hold onto the family she had left.

And Zac had had a life-changing revelation about his origins—he too needed the stability of their sibling connection. Mimmi was the only biological relative he had ever known and she was helping him through this difficult transition.

So Evie had accepted the other girl as quickly as she could, and grown to like her more as time went on. Mimmi was inquisitive, and very sweet. They had even started hanging out without Zac sometimes, and Evie was beginning to feel as if she had almost gained a sister too.

She heard Zac draw a deep breath as they approached the door and she gave his hand a reassuring squeeze.

She didn't think that he needed to be quite so nervous. Evie was sure that Mimmi would love her present—she had helped Zac prick out a silver moon bracelet, to match the necklace their mother had given her as a child.

It was absolutely perfect for their first birthday together.

Zac knocked and, seconds later, the door to Rita's house flew open. A blur of black and tan excitedly bounced outside.

"Zac!" Mimmi proclaimed, throwing her arms around her much taller brother before moving on to pull Evie into an embrace as well. "Evie!"

She could see that Zac was grinning widely, as happy to see her as she was to see him.

"Come in! Rita has a huge plate of prawns waiting for us, and she said that she is making something called cake. Have you heard of cake? Apparently land people love it."

"Sounds great." Zac replied with a chuckle, throwing his arm around his sister's shoulder and guiding her back through the doorway.

It was sweet to see him act so gentle and protective around her—he had taken on the brotherly role well and Evie smiled as she watched the exchange.

She was willing to bet that Mimmi would enjoy her first land birthday quite a bit.

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(CAM)

It had been a rough, lonely, few months.

First, Zac had vowed never to forgive him for his moment of madness on Mako. And Nixie had left. Then, Evie had joined the fish club and some new mermaids had shown up to create even more drama in the small seaside town.

Cam had apologized countless times, and tried to show that he could still help his former best friend deal with the chaos in his life—but, for the most part, Zac had remained stubborn.

For a little while, he had thought that he was making progress—thought that he was starting to regain a sense of comradery with the dark haired merman.

But then, Zac had had his big revelation. He'd found out that he was a real live honest to goodness fish-boy and he had spent most of the last month hanging out with his newfound sister.

Thankfully, at least, Zac had not gone back to being quite as cold as he had been before. He didn't seem to hate him anymore—they'd chat every so often in the hallways at school, and wave at each other on the beach— but he was, apparently, far too distracted to focus on rebuilding his friendship with a mere mortal.

He couldn't compete with a long-lost mermaid sister.

Zac had a new partner in crime.

With a soft sigh, the boy took a sip of his pineapple juice and let his eyes drift across the café. Before long, his gaze fell upon the individuals in question and he could not suppress a small scowl.

Zac and Mimmi were huddled together at a table across the room, speaking softly as they examined some sort of old looking map.

"Might as well be Siamese twins." Cam muttered, jamming his straw into his drink a little too roughly. The glass made a clattering noise, and he watched as his old friend looked up from his mysterious project.

The dark haired boy gave him a small wave and smile, which Cam returned, before returning to whatever he and Mimmi were doing.

It was progress, at least.

But Cam knew that he could never really be part of their world.

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(RITA)

She could hear the sound of bickering voices as she walked down the stone steps into the grotto.

It had been a long day of staff meetings and student discipline at the school, and Rita had to admit that she was not thrilled to hear the sounds of yet another conflict.

"I think it would be silly to swim all the way there." She heard a male voice say. "It's so far, and we don't have any reason to think that she would still be there after all these years."

She was quite sure that it was Zac, and she was also quite sure that he knew who he was "debating" with.

"But it's also the only lead we have." Mimmi replied, confirming Rita's suspicion. "That's the last place anyone saw her—I don't know where else to start."

"Aren't you supposed to be amazing at spells? Can't a spell help us?"

"Maybe, but that's probably not the sort of thing I can cast alone. It would take a lot of magic."

"Well, I can help—oh, hi Rita!"

Zac stopped in midsentence as he caught sight of the new arrival and Rita greeted him with a warm smile.

Even though she had only heard a few snippets of their conversation, she was quite sure that she knew what the two siblings were talking about. She had found them in here, surrounded by books, several times in the last couple of weeks—and when she had asked them what they were doing, they had admitted that they were trying to find a way to locate their estranged mother.

Sympathetic to their cause, Rita had given them what little information she could. But, being a southern mermaid, Nerissa had been little more than a legend to her— the powerful mermaid from the north who had defied her pod and mingled with mermen.

"Still working on it?" Rita asked, finishing her descent into the grotto and taking a seat on the stone ledge.

Both Zac and Mimmi nodded, their brows furrowing in a nearly identical way.

"No real leads." Zac stated, giving his sister a pointed look.

Mimmi sighed and rolled her eyes before picking up another spell book from a pile on the table.

"Maybe a spell is the right way to go." Rita suggested helpfully. "I'm pretty sure that there is a "call a lost mermaid" spell on one of my books—it would be complicated but, if anyone can do it, you guys can."

Zac, who was still rather new to the concept of northern mermaid magic, looked skeptical. Mimmi, however, nodded with determination.

"You have a point." She said, running her hand through her dark hair as she opened the book to the table of contents. "It can't hurt to try."

Looking a bit more encouraged, Zac grabbed one of the books off of the table and began to flip.

For a moment, a blissful silence filled the room and Rita watched the two siblings set about their work, a sad smile flickering across her face.

She had meant what she had said—despite their own doubts, if anyone could cast a powerful spell and find Nerissa, it was them.

Perhaps she had been imagining it but, as the two had spent more time together during the past several weeks, Rita had begun to sense a growing magical aura around them both. They all knew that their visions had started when the two had first met…but perhaps even more untapped powers would begin to surface as their bond grew.

A year ago, Rita wouldn't have believed that any of it was possible— she had thought that the northern mermaid's powers had been exaggerated in tales, and she had been skeptical that her offspring even existed. But now, as she watched them set about their work, she somehow knew that they were stronger together than she could have ever imagined.

The son and daughter of Nerissa might one day be unstoppable.

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(NERISSA)

It had been a long time since she had come this close to land but, even though she was not quite ready to reveal herself, she had been unable to stay away any longer.

She may have been powerful and feared, but her heart was not made of steel (no matter what some might say). They were her babies, and she was their mother. She had missed them dearly ever since they had been torn apart.

Nerissa had known for a while now that Zac and Mimmi had been reunited—she had felt their magical bond ignite the moment they first (re)met, and she had been tracking their movements from afar. She had known where they were, and it hadn't taken her long to find them.

But sensing them from a distance was not the same as finally seeing them in the flesh. When she had first peered above the water (invisible, of course), the sight of them had nearly taken her breath away.

They were sitting on the dock, outside of the house where Nerissa had left her precious son nearly two decades ago. It had pained her to do it, but she had not been given any other choice—mermen were not allowed in mermaid pods and, if the others had found him, he would have been destroyed.

As much as it hurt, she knew that she had made the right decision. He had grown up safe here—loved and accepted. No pod would have offered him that. Their silly feuds and prejudices refused to die.

And now, at least, Zac had found his sister. The darling girl that she had only been able to raise for a few short years before she had been driven away.

The faint sound of laughter danced across the water and, despite her sadness, the mother could not suppress a smile. She could not hear what they were saying, but it warmed her to see them getting along so well. Although the sun was starting to set, there was still enough light for her to make out the happy expressions on their faces. Mimmi was just as beautiful as she remembered, and Zac had grown into such a handsome young merman—just like his father.

She could sense, even from here, that they had both inherited her magic.

Nerissia heard Mimmi let out a small yelp of surprise and, for a second, she worried that she had let her invisibility falter. A moment later, however, the mermaid realized what had elicited the sound.

An unexpectedly large wave had lapped against the dock where her children sat, splashing them both with flecks of water. She watched as their tails appeared, almost simultaneously.

The sight brought yet another smile to her face. Seeing them with legs had been a little bit unnerving—unnatural. Nerissa was pleased to see them both return to their natural forms. These were her babies, just as they were meant to be.

To her, they were absolutely perfect.

As the water calmed, the woman caught a brief snippet of their conversation.

"Might as well go for a swim." She heard Zac say as he began to pull himself towards the edge of the dock, his blue tail glistening in the setting sun.

"Sure. It's a great night for it." Mimmi replied, grinning warmly.

As the two splashed into the water, Nerissa instinctively drew back. Even though she was currently invisible, there was a chance that they would see a disturbance in the water or detect her presence. They were powerful, and it wouldn't take them long to sense another powerful force.

Reluctantly, she ducked beneath the waves, trying to sear the last image of her beautiful children into her mind's eye.

She wished that she could go to them.

Embrace them.

Take them home.

But she knew that it wasn't time. Not quite yet.

Nerissa both anticipated and dreaded that fateful day. She was certain that neither would forgive her easily but she hoped that, one day, they could all find a way to be happy again.

Everything she did, she had done for them.

She had done it for this moment.

And she hoped that Zac and Mimmi would understand.

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A/N—Second one-shot in one day. Had to get it out of my head. Let me know what you think!