well you certainly aren't going to kill yourself to make her happy!
Listen honey, you are a survivor.....you fought CANCER......as a child, no less....You are a strong strong girl with your whole life ahead of you. Don't be sad about your sister...there was nothing you could have done.....you got sick, your mom was there for you.....the way it should be, end of story!
you have nothing to feel guilty about. You are not the villian. In time maybe she will realize how wrong she is, and maybe she won't......but don't let it affect your state of mind for one minute! you are a survivor! you can certainly weather this storm!