Darling, do not apologize, because you have nothing to apologize for. I don't have the right combination of words to make this better, I don't know that there even are such a thing. I know what it is to be left behind and wondering. I know what it is now to love you both and bear witness to your struggles, and that is difficult enough.
I'm not certain, as you've said, that there is anything you can say or do to make yourself feel alright with what your brother has decided. I think, however, that the first step is forgiving yourself. Much easier said than done, yes, but darling? This is a weight that you cannot carry. Nor should you. It isn't to make light or negate what's happened, it is to face it and accept it and allow for something else to grow.
You don't have to shuffle through it all now. No one expects that. Give yourself time, Vex, be patient with yourself--which, actually, is advice I've given your brother so now I've come full circle and isn't that an interesting coincidence.