My sister sleeps with her head on my boyfriend's lap while I'm asleep

I have a boyfriend for some years and he is pretty serious about it. We live together and it was always great. However, last year my little sister moved in with us because of an internship a couple of hours away from home. She is 20 years old and I am 29. I love my sister a lot and it has always been fun having her around. She lives with us since February 2023. In the beginning, it was great. However, I have the feeling that my bf is a bit too close with her. At first, I thought it was just friendship but the more I witness moments where he "doesn't see her as a kid" the more I get concerned. <br><br>For example, I always change clothes in the living room and leave my shirt open for a minute. I don't worry since my bf and I have been together for years. However, my sister sometimes walks in on me and stares. He just laughs about it. <br><br>I sleep naked in bed and go to the bathroom in the night. One time I returned from the bathroom without clothes and my sister was in our bed as well (I didn't know). My bf and I were sleeping, my sister just joining us and he wasn't aware. He laughed and said I should have locked the bathroom door. This laughter is something that I found really inappropriate.<br><br>Then, my sister joins us in bed almost every night to cuddle with us. During the day she lies on my boyfriend's lap while I am in the kitchen or working and he is watching a movie. I told him I'm not so comfortable with it but he said I should stop being jealous. I told him I just don't want my sister to get the wrong picture and fall in love with him. He said there's nothing wrong with that type of cuddling. <br><br>Last night I woke up at 4 am and saw my sister sleeping in bed with us. I asked "what the hell" and my bf said she came in at 1 am. He usually falls asleep at 11 pm but I had to work longer and went to bed at 1 am. I asked if they were hanging out and he said that my sister came into bed at 1 am and put her head on his lap. Then he fell asleep. I woke up at 4 am and her head was still on his lap. I shouted at him and he said I'm being a psycho. He said he was asleep and it's not his job to police my sister. I left the house, slept at a friends and won't return today.