bf left my niece/his uncles step-daughter for me

I've been with my bf for just over a year now. I found out, recently that he did in fact leave my niece for me. They dated for about a year, was mostly a drunken fling type thing. He mentioned it in the beginning but was vague, I heard rumours but didn't think much of it, back then.
He just stated he stopped seeing a female for me, I didn't know who she was or what type of relationship it was. Nobody really knew about them, just their close friends. His uncle and his wife, who is my first cousin didn't know and they still don't.
She was trying to hook up with one of his cousins and tried to keep seeing him, he claims she was starting to get too insecure, jealous, he met me and stopped seeing her.
I have her on social media, because she is my family, she does seem jealous about it and is bragging she was "secretly banging him" last year. I'm not jealous but just hard to get past it, to me, seema gross? and crossing a line to sleep with an uncles step-daughter?
He also used to brag, a lot, in the first 4-5 months of who he had, who wanted to bang him and seemed bit obsessed with one ex, he bragged he would have had a 5 yr old son with her, had a year fling with her too, met her at work, started "messing around" with her.
And in the end, she left him for his cousin. One female he bragged about is his best friends gf, he was all proud that his best friends gf actually wanted be with him first, not sure if anything happened but she ended up settling for his best friend.
How do I let go of all of this? Am I wrong to let it bother me?

If it was me I'd let HIM go along with all of this weird drama he brings with him. Anyone who brags about who they "bang" isn't worth having as a partner imo.
that is just how I feel I am sure a lot of people wouldn't care or be bothered but seems distasteful to me. You are allowed to feel bothered by it, I'd also remove your niece off your social media if she is saying those things and she knows you are dating him now, she is bitter and knows you'll see what she types.