Don’t try to dictate what she should write about. I kind of agree with her – it IS a downer. The prompts shouldn’t be taken that literally – the goal is to make them want you on campus. When they come to the table to defend her as a candidate for admission, do you really want the shorthand description to be “the one with the brother with the degenerative disease”?
Have her read this if she hasn’t yet: http://talk.collegeconfidential.com/college-essays/2003258-essay-tips-to-consider.html#latest