salukigirl, I think you cannot solve your sister's problems. You can forge/mend your own relationship with your father and his girlfriend. You can choose to see your father's girlfriend or not, or choose just to see your father. You can also choose not to spend your Christmas at your sister's. If she doesn't invite your father to her house, maybe you would rather also not attend and instead have a quiet get-together with your father over the holidays--meet at a restaurant for lunch one day, whatever you are up to. There are lots of options. Do not get sucked in to any sort of grudge or competition your sister is holding. Also, why don't you volunteer to hold Christmas next year at your place and then model how you would like it to be done?

I am in the midst of a divorce right now and my husband cheated on me for quite some time. He is still with his latest girlfriend. If he stays with her, there is going to come a time when she will meet my daughters and perhaps have a relationship with them. So it goes. People get divorced and move on and have different partners. It is hard for the kids to accept that, but their parents aren't going to be hermits the rest of their lives. I don't believe that "the other woman" usually breaks up the marriage. My marriage was bad before the affairs began. The affairs my husband had were a symptom of the bad marriage, not a cause. The best thing for us was to split up, affairs or not. Also, your father's girlfriend may have been keeping her distance from you at his request. He may have been trying to protect you, but to you it appears she was not interested. Who knows. Perhaps you can share the thought with your father that his girlfriend seems uninterested and see what he says. It's all so complicated. Best of luck to you and your sister. Holidays are hard.