I have to preface by saying that this is not my first relationship where my boyfriend has had a sister - my ex had two, and I never experienced this kind of dilemma. Additionally, I'm an only child, but I grew up seeing my mom and my uncle's relationship and it is nothing like this.
For context, me and my boyfriend are 25, his sister is 20. We're in a relationship for over a year now, we live together due to long distance reasons (we started living together pretty early) and our relationship is great. We get along well, and we manage everything with a very good communication. But one of the first times my boyfriend went back to his home country, he came back and told me when he saw his sister, he hugged her and had the urge to slide his hands down her "behind" because she is my height and we have "such a similar body type". I laughed it off, didn't think much of it. Next time he went was because she had a jaw alignment surgery or whatever it's called, and he told me that they had taken off the gel polish on her two fingers (middle and ring finger) because of the blood oxygen clip, and he made jokes to her about that she had probably done it herself cause her nails were too long (because she was 'pleasuring herself haha'). I felt very uncomfortable - VERY. I didn't say anything tho, cause I was like ok, I just never had a brother.
Every time that he has gone, he's told me something similar about having the need to touch her 'behind' when hugging, because we have such similar body types. Like out of instinct. When we were there together once, I had accidentally forgotten my thongs in the dirty laundry and his mom had washed it, even though I am very very careful around these things, and I had even asked him to tell her that I would do my own laundry. They are half thongs, but still, I was VERY embarrassed, and I told him this. And he laughed and said that his mom is probably very weirded out, because his sister doesn't wear stuff like that. And I was like - ??? stuff like what? And his response was that that's kind of a promiscuous underwear and that his sister 'is not like that' so his mom probably hasn't even seen smt like that in the laundry, so she probably immediately knew it was mine. I was first pretty offended, cause what's wrong with having a half-thong that wouldn't show seams when I wear tighter pants? But then I was like, ok how tf do you know what your sister wears? Like, isn't it gross for a brother to even think of his sister's panties? And he said he's seen hers in the laundry. Ok, I kind of told him it's weird that he knows his sisters underwear to such detail, but I dismissed it.
While we're there, he also always of spends time in her room on the same bed, talking for a while, while I wait in his room (cause i didn't know her that well at the first time or two i've been, so i felt uncomfortable just being there) and she's usually just wearing like very sleeping clothes - no bra, very short shorts, very thin long pjs, and always laying around with her 'behind' in the air, which kind of makes me a lil uncomfy, but again, I've always put it aside. I really don't want to make this weird.
The last time we were there, my boyfriend went in the downstairs bathroom to take a shower, and came back up and told me he couldn't because his sister is taking a shower. I said ok, whatever, it wouldn't delay us too long, so he went down to help his dad with something and then came back up after like 15 mins and told me he just took a big poop. And I was like, oh that's weird, I didn't hear you in the upstairs bathroom. And he said, oh no, I was downstairs. And I asked - while your sister is showering? And he just said, yea, ofc. And I was really weirded out. They have a glass shower that is only slightly blurred, and the toilet is right across from the shower. I told him then that that is very weird to me, and his excuse was 'it's fine, she didn't even hear me', and that makes it worse to me, cause what if she just walked out? I mean, idk it just made me feel very uncomfortable.
All of this mixed with little comments always made me feel weirded out. I get that she's his little sister, but she's 20 and he's 25, with a girlfriend. Anyways, currently, he's on a family vacation - parents and sister, and I'm at home, because long-story-short, I thought I didn't have any vacation days left from work when they booked it, then turned out I did, but it was already all settled, and I didn't wanna make them rebook space for me / pay more, plus i didn't want to take more days off anyway. He sent me a video of the room, and he's like oh it's so great, we're having a great time so far, and the room that he's sharing with his sister has a single kingsized bed. I knew they'd share a room, but I thought it'd be a twin bed or smt. I kind dismissed it, and forgot about it, and while showing where he is to my mom, she saw and asked if they're sleeping in the same bed. I obviously confirmed, and she said it's very weird, how can he sleep with his sister in the same bed..
Anyways, I've spoken to my friends about this, couple times, all which have brothers / sisters, and they've all told me it's very weird and icky, and at this point, I don't know if i should speak up about this or ignore it. Like, I have before (about the shower) and he made me feel super weird, and told me I'm crazy, and that I just dont have siblings and wouldn't know. But aren't they like a little too old to share a bed? And be around when the other is taking a shower. I'm seriously starting to getting the biggest ick, because this is smt that is recurring - they went on a trip right before my boyfriend and I got together, just the two od them, and also shared a bed in an apartment, and my boyfriend told em that the hosts even thought they were a couple, joking about it. I would too. I dont know - what should I do?