My ex, her family, my family.

In 2017 I was in a relationship with this girl I know since preschool. Had one night with her sister (so got a second sister, too) 10 years ago as well, but haven't got deeper because they were (are) known to be Maneater and the prettiest girls in town. My ex was aware and ok with this. Anyway. On one day she texted me, that she has depression and can't see me right now. I was devastated. I missed her and I missed her daughter. 4 weeks later she texted me again. Say, how much she miss me and whatnot. Naive me startet to meet her again...until 4 weeks later. Almost the same text. I was devastated again, but got that it's over now. I'm pretty sure she fucked every cock grabable and cheated on me regularly. After that I was confronted from various people about topics I never spoke about in public. I'm pretty sure to know what that means. 2 weeks later my niece, who is practically my little sister, because we were raised together and are only 7 years apart knew what has happened. Our whole youth and her young adulthood (I'm almost 30) we had a close relationship. 1 week after the final breakup my niece sits with my ex in a bar, laughing, taking shots, dancing. My best friend sent me a video of them. They knew each other superficial back then. Now my niece works with her. My ex sometimes said mean things about my niece, when we were still together. I defensed my niece and said to my ex that she should tell her in her face instead of saying such in front of me and more importantly behind her back. Since my niece started her new job I completely broke contact to my her. I don't want my ex to know the slightest bit about me, I don't want to know the slightest bit of her and I just can't forgive my niece. I just can't and nobody in my family seems to understand my point of view. They even say my point of view is "pathetic" I don't get it? Is my point really that off? I still love my niece to death, but no one this close hurted me like she did. I don't know how to go from here. I can't even talk to my niece and show her my point of view in fear that she tells my ex and my ex tells the whole town. My parents had a well known store and it's important for them to have a good reputation.

Just one or two things more: a few days after the breakup i found an old picture of my dad and a woman I never saw in my car. Probably an affair of his. My ex probably stashed it there so I can learn things I never wanted to know. I asked him about it and he denied it ofc. Oh, and her mum fucked my grandpa (!) when he was 60 and she was 20 or something. Someone told me and my dad confirmed it more or less.

I'm sorry this post is chronological not well ordered and I'm sorry for my (probably) bad English. Feel free to ask questions if you want.

Edit: typo