AIO? Should I leave my boyfriend?

TL;DR: There have been things that have lead me to believe that my boyfriend is attracted to his sister, I don’t know if I’m overreacting and making things a bigger deal in my head than they actually are or if there’s no doubt about it. He treats me really well, better than anyone has ever treated me. We have a pretty perfect relationship aside from this and I want to make sure I’m not being crazy before I decide to end things that are so good. I thought I was going to marry him.

I 26F feel so insane, I need help before I figure out what to do. I’ve been with my boyfriend 24M for two years now, officially for a year and a half. I feel like I’ve had this feeling that he’s attracted to his sister and I don’t know if it’s just that or if they’ve even potentially done anything sexual before. I think every time I’ve felt this way I’ve just completely ignored it or pushed it to the back of my head because the thought of it is insane to me, it makes me feel crazy. But I just started reading this book that talks about a man that couldn’t help his sexual attraction to his sister, and it brought all of these feelings flooding out and I feel like everything I’ve been suppressing and ignoring came to the front of my head and I feel sick to my stomach at everything I’ve ignored.

When we first met I thought he talked about his sister weirdly on 5-10 occasions, when he mentioned her he would bring up the fact that she attracted bad men because she had huge boobs and a huge butt and she had bad judgement and he thought a lot of the men that got with her were just using her/sexualizing her because of that. Or he brought up that she gets a lot of likes/followers on Instagram and TikTok because she’s attractive (she is extremely beautiful) and because of her big boobs/butt & she shows them off. It threw me off a bit because of the amount of times (it was excessive in my opinion) he mentioned how huge her boobs were or that she has a huge butt but I excused it away because of the context of the conversation.

Then once we were laying in bed naked talking about what our ideal body type was sexually and his response was “my sisters body type because she has really low body fat/she’s in really good shape.” I told him this was really weird for him to think of her in this moment and he said obviously he didn’t mean her, he just meant she’s in great shape so someone who is in good shape.

After this there were a couple other times I felt uncomfortable. One time I bought a bra and was surprised because it was a D cup and my boobs are quite small. I told him this, and he said “there’s no way you’re a size D my sister is a D cup and your boobs are way smaller.” Then a different time we were talking about who’s butt is bigger, mine or his. And he randomly says “my sister has a really big butt.” I told him this was weird and asked why he said that, and he said she’s the only girl he know’s that he could compare it to.

I also asked him early on into knowing eachother what porn he looks up and one of the first responses was “step sister.”

Another time he was doing pull-ups and his sister and I were both there and we weigh about the same weight and he told her to hold onto him while he does pull-ups, so he could see if he could do it with her weight on him. I thought it was weird that he asked her and not me.

He’s also really into the gym and getting bigger and I told him once that he doesn’t need to be huge to be attractive, and most girls don’t care about that. He said “my sister always says she wants a huge guy.” I asked him why it matters and he said “I’m just saying some girls do care about it.”

One time we were in his living room and his sister was in her room above the living room having sex with her boyfriend and we could hear the creaking and he got really agitated and put the pillow over his head so he could not hear it. I asked him why he was doing that and he said it’s really uncomfortable to hear your sister having sex.

Another time we were sitting on the couch and his sister was sitting next to him and their toes kept touching eachother on purpose it seemed like, I don’t know if this is weird cause if I was sitting next to my brother and our toes touched I would have moved them but it felt like they were touching on purpose.

One time when I went to his house I found a pink eyemask in his room and asked who’s it was, he said he didn’t know it was probably his sisters and he probably used it cause he couldn’t find his one night. Also I left my underwear at his house in his room and he said a couple months later that he lost them (were long distance and hadn’t seen eachother for months) and that his mom probably grabbed them when she did the laundry and assumed they were his sisters and put them in her stuff?

All of these situations are over the course of two years. I feel confused because he’s such a good boyfriend, he is so thoughtful and sweet and loving. We don’t have any problems aside from these thoughts I’ve been suppressing.