This is how I would handle it. Sister is a grown woman, it is not my place to admonish her for her behavior, what went on between her dh adn her during their marriage is between them, it is not between me, our parents, or any other siblings. She would always be welcome at my home with her new partner whether it be a man or a woman. However, in this case I would ask that before she bring her girlfriend that she formally introduces her to the other members of the family, I would not want that to happen at my house especially during the Holidays, in case there are members of the family who may have a problem with any aspect of the new (or not new) relationship. If after everyone met the girlfriend and sis was still comfortable bringing her I wouldn't have an issue, but its her responsibility to make sure everyone else is comfortable, not mine just because its my house.
ETA I don't consider what your sister is doing now as cheating. She does not live with her dh, the marriage is over whether or not she is holding a legal document stating it is. If she was still "married" and living with the dh that would be one thing, but she isn't cheating and its not my business whether she was before the marriage ended, its between her and her soon to be legal ex.