My bf and his cousin

My bf lives with his aunt and uncle and they have a daughter she’s a senior and my bf is 20. When I started dating him I noticed she would say things that made me uncomfortable. She said she wasn’t wearing a bra and underwear one time when we were all together..another time I went over and she just has her thongs anywhere and she also told him one time she wished she was dating him and my bf says that she was joking but still who says that?...Also my bf takes her where she needs to go. Cause she doesn’t wanna learn to drive. One time my bf sent me a pic of her and it was cause he said he was with her but the pic was of her back and she was wearing really tiny spandex and her ass cheeks were hanging out...and also she told him that when she had sex with her bf the condom broke, and today my bf got a new truck and he told me that she texted him that when was he gonna take her to get some ice cream in his new truck..idk if I’m exaggerating..but these things make me hella uncomfortable..I told my bf and he said I’m crazy and to get over it and that he doesn’t control what she says and he says I’m stupid. He says he sees her as his little sister...but I don’t see his actual sister talking to him like that...or in any other brother sister relationship. This seems to keep coming up. What do I do?