See if you can balance the stress; look for something that doesn't stress you, something you "like" and do that? When I'm having a hard time I read.

Beating a "dead horse" (your sister) doesn't get you what you need. http://www.humorbin.com/showitem.asp?item=266 She and your mother are obviously not reliable or helpful to you. Don't keep turning that way or you'll keep being stressed and disappointed. Ask the pdoc or hospital or other public agency about transportation help getting to your appointments.