I feel like my bf and his cousin do stuff

Idk why I got this feeling a while ago that they’re doing sexual things and I brought it up to him super nervous because it’s a pretty serious thing. There’s been stuff that has happened that makes me think that and it’s not normal behavior. They live together with her parents and during this quarantine thing I’m freaking out. He obviously denies it but they’re relationship isn’t normal. Since day one when I met them I noticed.

Some examples are that she told him in front of me that she wasn’t wearing underwear or a bra. Another is that he told me she used to barge into his room when he was naked..

Another is that she called him a cutie and said she’d date him because of all the places he takes me.

There’s more stuff but idk what to do, there’s no way for me to know for sure and well my bf has lied to me before about other stuff and I don’t really trust him when he says no especially with me seeing how they are.