Disagreement over the guest list caused a rift in the family!!

Our family is giving my mother a surprise 80th birthday party. Although the idea was conceived by my brother and sister in law, we are all contributing money. I have another sister in law who has alway been contentious and caused many a family rift. In fact, she has not participated in the family in years and just months ago was very rude to my very forgiving and loving mother. My mother would have no problem with her being there. This is the problem: The two sisters in law have recently become "friends" again. She is encouraging her strongly to come to the party. When others in the famly whom she has caused problems with over the years found out, they were not in agreement. When asked my opinion by the other sister in law I stated that the door for her to come could not be closed due to that fact that she is married to my brother (who by the way didn't think it would be a good idea for her to come), however, to encourage her beyond that would not be a good idea given the tension between her and others in the family. I also said this would make the party less about my mother's special day and more about the tension in the air. Please note that I live a ways from family and have no personal issues with this person. It was not personal. Also note, that she has not attended "any" family events in years and probably would not come, if not encouraged. Was I wrong to be truthful when asked? Now I am being alienated by the other sister in law for not agreening with her and giving the "Christian" answer.