Okay, this letter makes me really conflicted about who it's to. Different parts make me think of different people. I'm also entertaining the notion that it's still for "brother", just because I don't think we know enough about him to rule him out.
I bet you know what that's like--in fact, I know you do. It keeps you awake at night, that anger that you can't let go of...all because someone drilled into your head a sense of inferiority.
This line just makes me think of Daniel. He seems like the only one who has the possibility of an inferiority complex. Jonas seems pretty self-assured (although that could be a mask) Gemma might have some sort of inner inferiority (as I recall, many people suspected that she was jealous of Bree for doing the ceremony as a chosen one.) But I just think that it somehow fits Daniel. He has the anger going right now, because he's tired of coming in second to what's good for Bree. He's finally getting to a point where he's saying "to hell with it, it's time for this to be about me and my life."
Knowing what you dealt with awhile back really helped me put things in perspective. It helped me make peace with that part of my past, because I realized that my sister was never the enemy. There are other people to blame for creating that situation, but I also blame myself for not even trying to do anything about it except act like a spoiled brat.
But your situation is different in many ways…it’s almost the opposite actually. Although from what you’ve told me, maturity was neither of your strong suits (so at least we have that in common). Still, I know that how it all played out left a really bad taste in your mouth that you’ll probably carry around for the rest of your adulthood (the same way I held onto mine to the point where it almost ruined what little childhood I could cling to).
The "knowing what you dealt with" part makes me think of Jonas. He's the only one with a painful traumatic backstory that we're aware of. Unless another story comes out with one of the other major characters I don't know how else to interpret it. Instead of losing a sister he lost his parents, which could explain her calling it the opposite.
Here the reference of adulthood is what solidifies it against being to Bree for me at least. Tachyon is definitely intimating that they're an adult now, and as we've seen from her correspondence, she doesn't see Bree as an adult at all (since she's too much of a little girl to take care of Daniel's manly needs

)
And yes, you can at least feel better about blaming yourself for the mess you got yourself into because a lot of it was actually your fault. (I may suck at sympathy, but writing it out really does wonders for my own issues LOL).
Again, smacks of Daniel. Jonas, as far as we know, didn't do anything that caused his parents to dissappear. It's a bit of a stretch, but Gemma could have "caused" her mess she's in by sending advice online and thus getting a watcher after her (since I don't believe that she just imagined it, but that's personal opinion on the storyline.) But the most obvious vote is Daniel, at least in my mind. He chose to help Bree run away, the posted videos of where they were, just watched as Bree slipped into a more and more childish state (although there wasn't much he could do about it, I'm sure he felt some guilt.)
All the rest of it is just confusing to me. Since it's speculating about things that she's going to reveal to the letter recipient in the future, I don't think it's supposed to make a whole lot of sense yet.
*whew* Okay, that's my take on it. I don't know what it all adds up to, but I think once we figure out who it's supposed to go to, that the interpretation will be 1000 times easier.