Fued between my fiance, and sisters

so the drama started about 5 years ago. my sister just started dating her boyfriend she met online. he seemed like a nice guy, my boyfriend at the time (now fiance) got alone with him well, we would FaceTime them and talk for hours. well one night my sisters boyfriend got mad at her and started flirting with me. I responded, I was under the influence and shouldn’t have. I more responded just to see what he would say. he ended up sending me photos of his private area as well as a video of him doing the dirty to himself. I told my boyfriend what happened in the morning. ever since that night, he has hated my sisters boyfriend more than anyone. he refuses to forgive him and says after all he said to him on FaceTime and how nice he thought he was, it felt like a slap in the face because he thought he had a brother in the family. he forgave us for the conversation piece of it, however he said he can never forgive the video. flash forward to now, my sister, her boyfriend and also my half sister and my half sisters best friend went on a family cruise together without us. after the cruise, my sister messaged me and told me that she found out from my half sisters best friend my fiance and I were talking smack about her and her boyfriend. first of all, we talked about the situation with my half sister and her best friend on FaceTime in private so I felt betrayed by her best friend to air out everything we spoke about on a cruise to everyone. we didn’t talk poorly about my sister just her boyfriend but it’s known we don’t like him. it sucks because now my sister is very upset with me and my fiance. she said if my fiance hates them then she hates him and she won’t be attending our wedding next year. she was kind and bought me my dress because we have no parents and she wanted to help our, but she said she will return it and cancel my bridal shower. I have 2 kids a 4 year old who is newly in remission from cancer he battled the last 2 years, and a 5 month old daughter who love her. it sucks because she is my only family member I am close with and we grew up together. we are only a year and a half apart. but as we got into relationships and now with men that don’t like each other it’s hard to manage. her boyfriend is also very wealthy and ever since she started dating him all she talks about is how much money he has, how rich his family is, how they can do whatever they want whenever they want, saying this all while knowing my son just got done with cancer treatments and we had to quit our jobs for 2 years to care for him and we are broke now. it just feels like a slap in the face sometimes. is my fiance right for feeling like he wants no part in them? I guess the only thing that would make me sad is if she isn’t at our wedding. we have no parents and barely any family so it would basically be my close friends and my fiancés family. i blocked all of my family that were on the cruise because I felt like they all just talked poorly about us once they heard about the situation and never got to hear my fiancés side, so now my fiance looks bad to my family he’s never even met.