I love this story and I am glad you added a second chapter to this absolute gem of a fanfic for these three.

Okay, you said "grammar check this for me" so here we go...

In the first line has a couple of mistakes: has is present tense, the rest of the sentence and story are past tense. It would be better to change it to had. There should be a "for" before the sales, it should be living rather than live, and it should be in peace rather than at peace.
Second paragraph: bother should be bothered. and it should be "would like them to" or "would like to admit".
paragraph 4: again, has should be had. And it should read "that was without".
5: "even if she would" ought to be "if she would even". It makes more grammatical sense that way. And with "but always hiding it from the other so as not to get in any troubles as the last time."... are you saying they were always hiding their fighting from her, or are they just grumpy with each other but trying to conceal their reason from each other? Also everything after the "so" is a little grammatically off but how to fix it would depend on what you're trying to say.
eg. "but they were always hiding it from her so not to upset her as they had the last time she saw them fight."
OR "but they were always hiding the motive from each other as they did not want their jealousy to come between them again."
6: will is future tense, it should be "would" as they are simply speculating at this time, rather than speaking of a definite future event. eg. "your birthday will be in three months" vs "it would be nice to have a big party" one is fact and gets "will", the other is thought and gets "would". Also, you should use "was" instead of "is" and "meanwhile" or "however" rather than "otherwise". "Otherwise" indicates something more like "A is going to happen, and if it doesn't, then you'll get B." "Meanwhile" indicates something happening at the same time that goes along a parallel thread and doesn't interfere with the first thing, while "however" indicates something happening at the same time that goes against the first thing and contradicts it.
It should be "First, they did a few tests," you left out the y.
"trying if they could" should be "seeing if they could". "by the fact of sharing" should either be "by sharing" or "with the fact of sharing", but the first is grammatically the best. "one day in their" should be "one day on their" or "one day during their".
"She thought a few seconds that were important while Hikaru and Kaoru were thankful that the other host were busy and weren't listening." ... this sentence is... erm... first, "host" should be plural. And I believe there ought to be a comma (",") after important, if important is even what's supposed to be there. But the stuff before "important" don't fit together very well. I'm not really sure what it is you are trying to say. Are you saying she's thinking it over because it's an important question to her that she feels deserves proper thought? Or are you saying the fact that she was thinking was important? Like, are Hikaru and Kaoru worrying over the fact that she's taking this time to think? Are they relieved by it?
eg. "Because it was an important question, she took a few seconds to think about it, while..."
OR "Haruhi became silent in contemplation, leaving the twins to fret for those precious few seconds over what her answer would be, only thankful that the other hosts were occupied elsewhere."
OR "She paused for a moment to think of her response, and they were relieved that she was actually taking their question seriously, rather than mistaking it for something like just going to the store, or brushing their question off as nonsensical. However, they were also thankful the other hosts..."
Also in that paragraph, "sometime" should end in an s, because it is something that has happened from time to time, without an s indicates it will happen at some unspecified time in the future, while if it has only happened once in the past you indicate it as "once". And the sentence needs a period at the very end of it.
It should be "Haruhi has always been very understanding" (the words were in the wrong order) and later in that paragraph "was obvious should they tell" flows better.
"Nobody spoke to her and she sat very still, already used to it, knowing it just had to be one of her host friends. What a sense of humor, she thought bitterly when they finally got her out of the car (a limousine?) and shoved her inside a… plane?" There were just a bunch of small corrections in that whole thing that felt easier to correct as one large clump rather than one or two words at a time...
"used to the brightness and the flight". Also, English should be capitalized, and the end of the sentence, "with the ones and only twins." ought to be "with none other than the twins." instead.
"above her head on his snowboard". "Come on! It'll be fun" not "Come one! It'll be fun". "Of her, probably." should be "At her, probably." "Is even more dangerous" should start with either an "It" or a "That" before the "is". "I don't know how did you dodge those people so fast with your eyes closed" ought to be "I don't know how you dodged those people so fast with your eyes closed."
"angry with them for much" (left out the m) also, the commas should be to the inside of the quotation marks, like this "bla bla bla," not to the outside like this "bla bla bla",
"guided to a hotel" "an" is only used before a vowel sound. "Neither saw the anxious faces of the twins" I think what you meant to say was "She didn't even notice the anxious faces of the twins".
"She stood in her bed, putting away the blankets, and listened carefully to confirm that it was sob indeed." "Stood" means to stand, so she could have stood by her bed, or on her bed, she could have sat or laid in her bed, but not stood in her bed. And there should be an "a" before "sob". "She smiled sadly but slightly amused ... were crying in each other's arms." and everything else between is correct.
"ad he intended in" should be "as he intended in", "and trapped her swollen lips with her own taking" the second her should be his, "because neither of them really" "neither" indicates two, it should be "none" if you want to indicate all three feel the same way in this situation. "What would they said to the" because they are contemplating on a future event, it should be in future tense (say) rather than past (said). And finally the last word in this should be "time", not "moment" as moment indicates less than a second.

Other than that, it wasn't bad. Perhaps a little fast, it would have been nice to see a little more of the twins individual thoughts, and maybe them discussing the idea of a threesome together beforehand, deciding that they both wanted it more than to never be with her or to see the other hurt because she was only with them. Get some idea of the real brotherly love between them, and make clear if you see their brotherly love as a twincest feel to it or just close siblings that feel the same way for Haruhi and are willing to share because of it. You wouldn't really need to bring up too much more than you already have, but rather the suggestion is to present it as a private conversation, rather than simply exposition.

A sequel wouldn't be bad. Would they tell their friends/family? How would different people react? Would Tamaki be thrown into some jealous rage and try to forbid Haruhi from ever interacting with the twins again as her "Otou-san"? Would Kyoya show concern and attempt to explain to them how such relationships would be frowned upon by society? Would he make a fuss about how such a thing would get in the way of club activities? Or would he want to exploit them and make a profit off of it? What about Hani and Mori, the "uncles" of the "family"? Or Range, should she find out? Or would they try to hide it, only doing things behind closed doors in the twin's house? Would flirting and secret caresses be noticed as anything out of the ordinary with the way the twins have always treated her? Would they grow more jealous as an oblivious Tamaki tries to woo their girlfriend? Would others suspect and realize what was going on between the three, or remain blissfully ignorant?

Anyway, I hoped this helped.

Oh my god!
That was so cute and heartwarming! Great job.

This type of story generally isn't really my type of story, but I have it a chance because the description was very good. After reading it, I can say that I'm happy that I have it a chance because it was very good.

That was cute and your command of English is decent. You tripped on the grammar but vocabulary is excellent. Will there be more to the story?

That was all cute :3 I like it !

I LOVED IT! I NEED MORE!

Ahhh yes! This was amazing!

AAAAH LOVED IT, PLEASE CONTINUE IT!