She can’t write it as a downer.
The focus should be that the opportunity to know her brother, to see all the wonderful things he is and can be, has helped her grow. It can’t be about family finances and all the things her brother can no longer do-- it has to be about the wonders of her relationship with him.
Remember, this is a sales job for her application. It’s not True Confessions, it’s a sales job. It’s sole purpose is to give them a reason to say yes to her application.