Shelly,
I can't believe it, this is just too much. But like others have said, God is big enough to handle whatever you feel right now, and you certainly have a right to feel it! Please don't give up. Your sister may have a much different road to travel than your mom and dad did, and it's MUCH more likely she can beat this awful disease. I also agree with the others that this might just mean it's time for some pharmaceutical help, if you don't have that already -- under a doctor's guidance, of course. You've already pushed yourself through so much. I won't try to give you a cheerleading message, because if I were you, I wouldn't be able to listen to one... but I do believe in you, 'cuz I've seen what you CAN do. You are in my thoughts and prayers, as is your sister.
((((hugs))),
BeckyCW