Key is to tell a story and to write an essay no one but you could have written. It’s all in the details you choose. Was there one moment in the whole experience that stood out? Maybe lead with that. Tell the story “in the moment” but reflect on it from a distance. If there’s any way to interject humor, do so. (I realize this is NOT a humorous subject - but usually there’s a moment or two that are just so absurd or LOL funny that it can provide comic relief.) Remember, it’s about YOU - not your brother. All that said, I’d encourage you to try writing about a totally different topic as well. Write two essays. Show both essays to a few people who don’t know you (or your brother) that well. See which one they like. A little market research, if you will.
If this does not end up being your main Common App essay, you can still work in pieces of the experience in many of the supplemental essays you’ll be writing for top schools. For example, if you’re asked why you’re pursuing a specific major or even why X school.