This piece is very purposeful, and has very powerful themes! LGBT suicide is a terrible problem, and it's good that someone is trying to tackle it. Here I see the seeds of a beautiful story about loss and hope. It just needs some more work to release that in its full potential.
One of the most problematic aspects of this piece the way it is now, is that it's very heavy handed. It comes across this way because readers introduced to Maria and her issue postmortem. We are given a letter, by a girl that we don't know. Her death is cheapened by us not knowing her beforehand, because we have had no time to identify with her. To really have her family and peer problems hit home, we need to see them. I want to see her parents reject her. I want to see her peers make fun of her. I need to see her have interpersonal relationships, and Maria herself before I can know her and care about her properly.
This problem is repeated again in the second half, when Maria's brother is speaking. There's no mention of him being trans before, or anything other than hetero-normative, and without a few hints it feel like plot borne out of convenience. Again I want to see him. I want to see him with his sister, and feel his challenges and his love for her. I want to see the looks they exchange at the dinner table when his body is still female. To really know him and his journey, to see how his sister and her death have affected him, I need to see it!
All of this can be fixed just by writing more about these characters. Don't be afraid to explore their lives! Parts of them might be difficult, but that's what makes stories about social issues powerful. This piece has so much potential, that it's almost bursting with it. Just run with it!