These holiday obligations sure can make us miserable! You always have choices - you can stay home despite what your father wants or you can go to your sister's house ... it really is up to you. If you decide to go to your sister's, why not let the past be in the past (because it really IS in the past) and give her a smile when you get there and say "Thank you for inviting me - I am glad to be here". Then do your best to get through the event in a congenial way, stay for a reasonable amount of time but not too long, and leave knowing you have done your part.

Plan something fun for yourself for the next day like watching a movie you love. Thank goodness Christmas is only one day a year ... that is what gets me through.