There are a few things I can't single out.
1. Sex with my father where she screamed "yeah, fuck me, I'm your whore" or "suck my nipples, bite them, drink me" I watched them having sex around 10-14 years old when I had the chance.
2.A couple times she didn't wear a bra when my friends were at home and her nipples were visible through her clothes, there was an occasion when my friends came over and we were playing console, she was sitting between them and her nipples were sticking out really hard through the fabric and my friends' eyes were directed there.
3.We had an inflatable pool at our country house, and one day my father's friends came with their wives, my mother wore a very thin bikini and was as naked as possible, her butt and breasts as a magnet attracted men, I was always ashamed that they looked at her, because other women were not so naked, for some reason I remember how men whistled at the pool when my mother carried beer)
4.Of course, the judo section, for me it was a turning point when she was groped a couple of times by boys, I wrote in detail here
https://bullyfucksmymom.com/fantasy-discussion/what-got-you-into-this-fantasy/90/, I had nightmares of them fucking her, making her their whore, and she was used by anyone who wanted, who disgusted me as much as possible, then because of these dreams a little later I started to get aroused, and that was the beginning of the fetish.
(Immediately I say my English is not perfect, my original language is Ukrainian).