I think my boyfriend has been intimate with his sister

We’ve been together for a year and over time things have just built up and up and made me far more suspicious.

When we first started dating he told me that they were super close and that’s fine like I love that, he’s 33 and she’s 30, the first time I met her she was cold and distant towards me but it was a first meet so it was expected to be awkward, we went to the beach the second time with her children and my bf and his sister both had swim suits on, as you’d expect and she was sitting on his lap.. which was weird to me but the weirder part was the way she sat down, she like turned around, stuck her bum out and back down onto his crotch and he took her by the waist and guided her down, then wrapped his arms around her, which I found creepy as fuck.

As time went on, I started to notice more, they’d give a hug and a kiss when greeting and leaving but the kiss would always be awkward, sort of half the lips, sometimes the neck, it was always uncomfortable to see, and the hug would be a little too touchy feely for siblings in my opinion, she stayed over one night for whatever reason and they both walked around in underwear and she come out the shower with his t shirt on and pants so I was absolutely creeped the hell out.

I started to dig deeper, looking at their photos together and every photo there is he’s got his arms around her waist, she’s kissing his cheek, there’s one with his hands around her boobs and both making a silly face, lots of her on his lap, one candid that someone took of her literally lying on top of him, just really intimate stuff. He told me he lost his virginity late at 26, and she said she was late too and said “yeah roughly same time I think”, they talk about sex a lot, like a lot, and she tries to tell me what she thinks he’d like. It’s so fucking creepy.

After 10 months of this (I haven’t had sex with him for about 6 months because I’ve been so creeped out by this) I brought it up, I should have sooner but there’s no gentle way to say I think you’ve fucked your sister hahahaha so I eventually told him a lot about their relationship makes me so uncomfortable and he went mental, screaming at me, saying I’d never love him the way she would and that she knows him better than anyone in this world etc etc which only added to my .. what the fuck… so I ended the relationship there and then, Bcos something was not sitting right, it was just too weird for me, too close for comfort, maybe I am totally wrong and they’re just really close but it just wasn’t for me. We only had sex twice and I couldn’t do it again, the whole thing just made me feel sick