While I generally do not encourage students to focus on this type of topic, reading this tells me that, for you, it is an excellent topic, and I think that you can do it very well.
In this thread, in a couple of posts, you have already conveyed a lot, have been entirely positive, even when writing about the challenges, and it is obvious that you love your brother.
So yes, I think that this is a great topic for you. You have the topic, and the material. Now you need to focus on the craft of writing.
So remember two things
A. The essay is about you, not about your brother
B. Show, don’t tell.
Go forth and write a killer essay.
Good luck!