Legacies are never guaranteed, what happens if she doesn’t get in where you wind up, will you leave? BS put a huge mental stress on students which can exacerbate the extreme issues she already has and cause huge set backs for students.
Are you picking a school for her best outcome, or for yours? Also, frequent medical appointments can be quite difficult to arrange with BS schedules as they are less forgiving for missing time on campus that a regular public or local private school may be. If you aren’t asking the admissions offices now how this would work in the future since you state this is part of your decision process, you might find yourself in a bind later when it comes time for your sister to apply. Of course choosing a school has to be right for the student AND family, but simply because it is near home where you can be a day student doesn’t mean it is going to be the right school for your sister and her needs, too. Being a boarder would alleviate the need for your parents to drive you everyday freeing their time to get your sister to appointments that she needs for her mental and physical well being.