a.n. from a prompt sent to my tumblr (bananannabeth). more headcanons than fic but thought i'd share it here anyway.

Percy is the proudest big brother ever. He brags to everyone, all the time. Annabeth and Grover are the only ones that can stand to listen to the non stop gushing.

He doesn't want to endanger his little sister, though, so he doesn't go to visit as often as he'd like to.

Sally and Paul make sure to keep him updated, still - They iris message every single weekend, and it becomes a game to see if he can surprise little Ava. Most of the time she's already there waiting for the call, jumping up and screaming either "Boo!" or "Percy!" as soon as he appears.

Sometimes it comes out as "Boocy!", a nickname which she takes a liking to. He gets referred to as "Big Brother Boocy" more often than he'd like, but he never says anything to her about it, because her smile makes the embarrassing name worth it.

When Percy and Annabeth do go to visit, Ava doesn't let them out of her sight. If she has to pee, Annabeth has to wait outside the bathroom for her, talking to her through the door the whole time so she knows she's still there. She holds onto Percy's hand, his sleeve, his leg, sits on his lap and fights for his attention constantly.

Annabeth convinces him that he should visit more often - that they'll be safe, especially if they stay around the apartment or make sure that the two of them are together whenever they take Ava out. He agrees, and when his visits become more regular Ava's clinginess dissipates somewhat. He kinda misses it.

When Annabeth hears that Sally and Paul have asked Percy to babysit, she reminds him that Ava loves her and she is head counsellor of the Athena cabin, so she is good at keeping children in line.
"You talk to them like they're just mini adults, Annabeth."
"You shouldn't talk down to children."
"No, but - you know what, never mind. Just trust me, I've got this."

Percy makes dinosaur nuggets for dinner and chases Ava around the kitchen with a t-rex one before she triumphantly bites its head right off.

They build a giant blanket fort in the lounge and snuggle in to watch Finding Nemo. She asks him if he can speak whale and when he assures her that he can she makes an awful howling noise right in his ear.
"What did I just say in whale?" she asks, giggling at how he winced.
"You just said, Percy is the best big brother in the world."
"No I didn't!"
He pretends to think. "My name's Ava and I'm a smelly little butt?"
"No! You're bad at this!"
A new voice chimes in, "Did you say, I wish Annabeth was here?"
Ava's whole face lights up and she rushes out of the blanket fort to tackle Annabeth in a hug. "Annabeth! I didn't know you spoke whale!"
His girlfriend's smile is entirely too smug for Percy's liking, until she says sincerely, "Percy taught me some."

They put on The Little Mermaid next. Ava is asleep in Annabeth's lap by the time Part of Your World comes on, but they don't stop the movie. Percy dramatically sings the reprise - complete with hand gestures and longing stare - to Annabeth, who has to try and contain her laughter so she doesn't wake up his little sister.

As the credits roll they tuck her into bed, safe and sound, and return to the blanket fort.
Annabeth nods approvingly. "This is well constructed. Bravo."
"That's high praise." Percy smiles. "Thanks for coming round tonight."
"Not gonna lie, I was a little worried about what I'd find."
"What, you didn't think I could handle her?"
"She can be pretty full on." Annabeth's smile is soft. "But of course I had nothing to worry about."
"Of course." Percy beams and puffs his chest out proudly.

They sleep in the blanket fort that night, and are woken up by a very happy, very loud Ava at 6am sharp.

They make sure Sally and Paul don't miss out on the fun by cooking them breakfast in bed and serving it to them at 6.45.
Sally reaches over her plate for their hands, smiling tenderly at the three of them. "We have the best kids in the world."