Am I being selfish and self centered about not wanting my fiance's ex wife to celebrate with us or am I alright?

My fiance wants to celebrate his daughter's 16th birthday together including his exwife. I'm not comfortable with it. I rather his daughter has two separate birthdays... one with her mom and one with us. I understand that they need to parent together but they dont need to gather together. I feel as if they need to start acting divorced. Is that selfish of me?
Am I being selfish and self centered about not wanting my fiance's ex wife to celebrate with us or am I alright?
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