I suggested a topic to my S that he was not thrilled with. But he wrote it and it was exactly what the essay needed to be. His GC was THRILLED. Sometimes 17 year olds don’t have enough life experience to see their own achievements or value in the broader context of their lives.
If your D believes that she is a particular person because of having this experience (with her brother), that she has learned xyz or has come to value abc, etc, then I think it’s fine. I’d limit discussion of her brother to a short first paragraph and then only refer back to it periodically to contextualize the value or perspective she now has.
One thing you might do is initiate a conversation with her about her life and what her values are, what’s affected who she is now. She may find herself talking about her brother’s influence. And then you can say voila. Take the college essay matter off the table and just talk.