Boyfriend is uncomfortably close to his sister?

I have been dating my boyfriend for almost a year now. As our relationship progresses, I feel more and more insecure about the relationship he has with his sister. For references, he has 8 siblings, 5 who were already married since he was young. Besides that is his two older brothers and his older sister. His sister has been taking care of him since he was young, as he is the baby of the family. They’d sleep together, bathe together, etc until middle school. His sister is a tomboy since she didn’t have her sisters to grow up with, she also has a strained relationship with her sisters due to some family problems.
In the beginning of our relationship, my boyfriend would often compare me to his sister about everything (Cooking, makeup, etc.) It did irritate me but I didn’t think much of it. Occasionally he would make comments about her body to me. I’m not sure if he’s seen her naked but he’d say things like “my sister has a good body shape” or “she has big boobs”, etc. I found it strange but brushed it off cause maybe they’re just close. My siblings and I are not close so I’m not too sure what is “normal” between siblings that are close. Eventually he told me that his family thought they were intimate because she would come to his room to play video games all night. He denied it and said nothing had ever happened. Putting everything together makes it seem more than possible for them to be intimate. Since then, I continue to feel insecure and disgusted by their relationship and don’t know if I’m being irrational or not.
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His sister also has a brocon, which makes me feel like there’s a mutual thing between them. She has been single for all of her life and he has often dated her friends. She also used to tell me about all the time they would spend together. She told me they would go out to eat, watch movies, and hang out all the time before I started dating him. The other day she said she wished there was a song about a brother’s love for his sister.
Boyfriend is uncomfortably close to his sister?
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