Don't think of therapy as a punishment for "bad behavior". It's a way to get to know yourself and to find out why you think and feel about things. I'm sure you feel some guilt over siding with your father over your sister, and that kind of emotion is very destructive. I hope your sister opens her mind and lets you in; maybe the two of you should attend therapy together. That would give her the opportunity to see that you're sincere. I hope it all works out for you!