have you seen this?
http://www.livingdonorsonline.org/kidney/kidney4.htmReasons I've heard for donors being turned down - any heart condition, high blood pressure, excessive drinking. I asked about mental disorders because most of my husband's family suffers from some form of anxiety disorder. They said they would be evaled on a case by case basis, and accepted or rejected on how severe the condition really was.
I know that if the donor wants to back out after offering, the medical team will fabricate a medical reason for them, so that there are no hurt feelings. I would think, if your sister wanted to donate, and you were hesitant because of her mental health issues, that the team would probably do the same in reverse - give her a reason why you weren't medically compatible, to spare her feelings when you turn her down.