Cheating and hurting people bothers me. If she had realized that her marriage was a mistake, explained the situation to her dh and son and left. And then later, even if it's only a little later she met somebody, I'd be ok with it. Even if she met the person and delayed pursuing it until she left the marriage and took it slow, because she has a son and a man she lived with for many years to consider. I would perhaps invite sis and her friend over for a small meal during the holidays but I would exclude the new gf from the family holiday meal. I'd even invite her soon to be ex over with the son. They are still family and the new gf could be gone in a short time.
Oh and this doesn't mean you don't support your sis. Tell her you love her, hug her. Tell her you are not condemning her choice. You just are concerned about the way she went about it and would feel better bringing the new gf around at a less intimate gathering.
It bothers me that so many people are acting hypocritical in this situation. I know very well that if it had been a man cheating on his wife with another woman, who announced that he was leaving his wife and son two months before the holidays and wanted to bring the new girl who he said made him happy to the family dinner, this thread would look much different.