What should I do?

My boyfriend has been staying with me for a couple of weeks until he moves into his own place. My mom hates him because he has been married and has kids that live away from him. Tomorrow is my cousin's wedding and my BF is supposed to attend with me. My mom told me not to bring him because I will upset my Grandmother, whom my mom has filled her head with nasty untruth's about my BF. Either I go alone and let my BF down, don't go at all and let me cousin down, or take him and potentially upset my grandmother...I don't know what to do. I told my mom that I w as going to call my Grandma and ask her if it was ok that he came and she said that I shouldn't because I will hear something that I don't want to hear...(My Grandma is very devout Catholic woman that doesn't believe in divorce.) What do I do???