Its not an absolute no-no to include that kind of content, you just need to be aware of the fact that you walk a very fine line when you do so. Remember that a PS is not just "why I want to be a doctor." Its "why I'm interested in medicine, and why I think I'd be good at it" I'm very sorry to hear about your sister, and if her illness was truly what first interested you in medicine I think it could be a powerful introduction-- if done well. However, remember that illness in the family can happen to anyone (god forbid), and does not give you any special credentials to be a physician.
After explaining how these negative circumstances gave you a POSITIVE view of the medical profession and peaked your interest, you should go on to explain what qualities you have-- not related to the random events of your families health problems-- that make you well suited for a career in medicine. You just don't want someone to read your PS and say, "well, he'd make a hell of a member in a support group, but I don't see why he thinks he has what it takes to be a physician."
In closing, you can use it if you use it right. Don't devote your whole PS to it, and remember your PS should answer two key questions:
1) Why is medicine right for you?
2) Why are you right for medicine?
Good luck