Christmas w/ recently divorced parents advise

Recently my parents have gotten a divorce after 30 years of marriage. My mother cheated on my father yet blames him for everything. This year is my brothers and I's first Christmas with divorced parents. My Mother's side of the family is having Christmas dinner the 27th or 28th -My brother's and I are invited. I kindly requested my mother that her "boyfriend" ( aka the guy who she cheated with ) not attend the party and it exploded into a whole argument about how horrible my dad is and that she LOVES this new guy. Am I wrong for asking him to stay home so that my Brother's and I can have a semi normal Christmas and feel comfortable?