AITA for saying I'm not going to her wedding after how she's acted towards me?

My Aunt Linda died recently. She was very close to me, I was her favourite out of all her children, nephews, nicces, cousins etc. I'm not close to my family at the moment but I still loved my Aunt a lot and she was so happy that I had gotten my life together.<br><br>I'm friends with a girl called Kat who was also close to my Aunt. Kat's been really supportive and understanding of me over the past few weeks, and she's been really helpful in sorting out the funeral. I was going to pay for it myself but Kat insisted on her and her fiancé pay for it as my Aunt paid for their wedding when they didn't have enough money. Kat's really upset because she was supposed to give out Arianna's (spellings not correct I know, small mistake sorry) programmes and she hasn't been given one, even though she was one of the flower girls at the wedding.<br><br>I found out last night that Arianna had been putting Kat down behind my back. Apparently when Kat got pregnant Arianna kept calling her a "whore" and a "slut" and made a comment on Kat's baby bump picture saying how she looked older with her bump than when she was the flower girl at her wedding. I'm so livid because Kat is an amazing mum and girlfriend and none of that is true. When Kat had her car broken into last year and everything was stolen, including Arianna's wedding gifts, instead of feeling bad for Kat Arianna kept saying how much the experience of her stuff being stolen ruined her holiday. Kat was really upset over the whole thing and said she'd rather have her money and belongings stolen than to experience that level of entitlement.<br><br>I've been really upset over this and I've spoken to Kat about it several times. I'm not close to my Uncle or Arianna anymore. I'm not going to Arianna's wedding and I don't want Arianna to know that I'm not going, but my family is pushing me to go. AITA for not going to her wedding after how she's treated Kat? I know Kat will be happy and she'll be able to enjoy her day more knowing that Arianna's not there.