It seems that your relationship with her has turned to contempt from you and perhaps the rest of the family towards her. You do not view her as a human being worthy of respect and dignity, because she is sick and does not respect herself either. I get it. I have been you and I have also been your sister. How would you like to be treated if you were in the same situation, a good place to base how we treat others. You feel bad already because you feel you've crossed a line, but it was bound to happen and is only a symptom of the truth that she embarrases you and that you despise her. She is better off not having family members do her any favors, and you can only escape your guilt once you come to terms with what you have done and perhaps change for the better or you may end up looking in the mirror and also holding yourself in contempt. Namaste.