advice??

So my boyfriend has a female friend who also happens to be his best friends sister. she’s around the same age as us and she’s pretty. He keeps going to her house and hanging out and some days getting drunk and staying the night.. he’s even ditched me to hang out at her place. i’ve told him it bothers me and makes me feel super uncomfortable and he says he doesn’t see a problem because she’s like a sister. and he keeps saying he won’t go back and then does and acts like i’m ridiculous for how i feel. He can have female friends that’s cool and fine but going to her house and drinking and staying the night?? and sometimes ditches hanging out with me to hang out at her place? it just really bothers me. what should i do? do i have a right to be bothered?