Am I wrong for refusing to attend my sister’s engagement party because she’s getting married to my ex?

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My sister says she understands, but I can tell she’s hurt that I’m refusing to be part of this big moment in her life. My mom is making me feel like the bad guy for not “being the bigger person.” I just don’t know if I can fake being okay with it right now.

Am I wrong for refusing to go to her engagement party?