History: My cousin L and I were pretty close growing up, but she was a spoiled brat and very self-centered. I was probably the only person that would not put up with her me me me! attitude, and I called her out on it a lot. Our mutual best friend, D, broke up with her fiance in March 2010, despite being completely in love with him. She did not want to live an hour away from work in a small town she didn't like (I don't blame her- white trash city, no joke). He had bought a house there the year before they met and said he wasn't going to sell it with the way the housing industry was because he wouldn't get shit for it. Instead of putting it up for rent like she suggested, and moving in with her, he gave her an ultimatum and she left him.
My cousin had known this guy for 10 years or so, and was infatuated with him. He was not into her, whatsoever. He dated all of her friends but would never give her the time of day because she wasn't his type.
My cousin was desperate to try to get with J, despite him being her best friend's ex-fiance. They ended up hooking up the summer of 2010. D stopped speaking to both of them, and it didn't even phase my cousin because, in her words, if D was REALLY her friend, she would be okay with it eventually. (I disagree.) J proposed summer 2011 to my cousin. She originally asked me to be a bridesmaid but I told her outright that I would not be a party to a wedding that crushed my best friend and that was a complete sham (I think J "settled" for my cousin because he couldn't find any other girls to put up with his shit). She got pissed at me. I found out later that she was getting into my Facebook account (she had my password). She found a message I had sent to my SO saying that she was acting like a selfish bitch and she's crazy for wanting me to be in her wedding.
We didn't speak again until our uncle passed a couple months ago. She was acting really nicey-nice and she's tried being on friend terms with me. However, I think she's a backstabber and I can't trust her. She said some pretty awful (and NOT true!) things about me and the situation, excluding all of the crap that she did not only to me, but to my and her former friend.
Am I being petty? I don't want to go to her wedding, so I won't. I have to go to my family reunion next month but would rather not speak to her.
What would you do?