So I find the idea of gaining an ability through their past traumatic event of losing a friend is interesting, but if that is what's happening I'm not entirely sure how that trauma created the dream power.
I'm also not very convinced that she would, after only experiencing the ability twice without any evidence that they were more than dreams, would actually believe it was a real power. I'd imagine a person would need to go through levels of convincing themselves before they say they have a supernatural power unless super natural powers are already a common thing.
Also I don't get why she went through the same dream twice one after the other.
And lastly, I don't get why they feel the need to hide their new ability. They don't know of any threats hunting them, and there haven't been any dangers brought up, so I don't get why this power would need to be kept secret.
Beyond those things, I think it's an OK read so far. Things unfolded pretty fast. A lot of characters were introduced one after the other. We got a flash back a school scene, two dream sequences and the declaration of having powers all in one chapter when I fell each of those things could have been given more time, maybe even their own chapters. There's not really any build up, but that's understandable because it's just one chapter until you look at all the cards being played in that one chapter. And it all flows well enough to follow but it just doesn't hit very hard.
I will admit I'm not a big fan of flashbacks in writing. I prefer, if you need to explain or bring in information that happened in the past, just make the scene take place in the past rather than having the scene in the present and expositing events. I think if you actually wrote the scene as it's own thing rather than a flash back, the death of her friend would be an even stronger opening to the story before jumping to the future where we see those events are still gripping. But these are very much just my opinions. I'll admit I wasn't invested at all until that part, but then we moved past it so fast (because that's what you naturally do with flashbacks) that what could have been a significant punch up ended up being a small jab.
I think the way you have it works, but I think it could be deeper.
One other small thing I will comment on is how you disclaim personalities a lot. Like you'll say
"he was always such a deeper thinker"
or
"she had a way with words that no one else did"
or such and such. I won't say never do this, because some times it fits, it works, it's effective. But rather than saying a person is a deep thinker, its better to have them say something that's deep and then let the reader learn how smart they are. And again, the way you wrote things work, but I noticed these few things enough to think they could be improved on if you feel like it.
But the one note I have that I demand you hear, the line where the youngest brother asks "do you want to play video games," was so unspecific it makes it feel stiff.
Have him ask to play a specific video game, it'll make him feel less generic and more believable.
Yes, kids like video games, but typically it's a specific game.
So something like
Hey do you want to play Sonic?
Or
Hey do you want to go play the new Streetfighter Mom got me?
Or
Hey do you want to play Mario cart?
Any specific game will feel more believable than just nondescript video games.
If this were me, I'd:
Make the scene where the childhood friend dies into its own chapter, the first chapter, and not just a short flashback.
Make the classroom scene and walk home it's own chapter and use it to show how losing her friend still haunts her, and better introduce the side characters because your current version doesn't have them say much so I don't really know how to feel about them aside from what you said are their personality traits. But I shouldn't have to rely on you telling me this person is smart this person is sweet, I should get that from what they say and do (and I kinda do but not enough for my taste)
Make the at home scene and the first dreams powers part its own chapter and really better introduce the younger brother and make the dream powers more unique and stand out.
Do a chapter where she confirms the things she did in her dreams were real, then have her decide to use it more, and give her a reason to need to keep it a secret.
That's what I would do.
I hope this is good feed back, but I feel like I might have gone overboard.