He wants his sister to move in😰😳

So I’m planning on moving in with my bf by the end of the year. He has a sister that’s in high school but she lives in Mexico rn. And he told me today that if we move in together that he told her she could come live with us.....I’ve never met her, I don’t want to sound rude but I don’t want her living with us. A teenager. We’re going to be responsible for her and it’s gonna be like having a kid. I’ve heard so many horror stories about living with family members. If I wanna yell at him for doing something stupid I won’t be able to, I won’t be able to walk around in my underwear and bra. Or have wild sex with my bf in the open. Like I’m gonna be constricted and I don’t know how to tell him. He was excited and it’s his damn sister. Plus he has another younger one as well and the line will keep going. I’m planning on moving out so I can be free to move around and do my own thing, because rn at home with my parents I’m controlled and constricted. He didn’t even discuss this with me before mentioning it to her. We’re both in our early 20s, How do I tell him.