Brother in law

Hello, so I need a little of advice. I am currently living with my fiancé in an apartment but our lease is about to be done. My fiancé's brother knitted that we are looking for another place to move to, he asked if he could move with us. Honestly I don't know what to do, I know it's his brother and don't get me wrong I love his brothers like if they were my own but I don't know if moving in with him would be such a great idea. We currently have a roomate and that's one of the reasons why we're moving. I really want my privacy and I want to do whatever I want in the comfort of my own home, he is still a teenager but he works A LOT so I wouldn't worry about him paying rent, but the only concern is that I don't want random girls/friends sleeping over and stuff you know? I know that he will be fine if I say no to moving in with his brother but I would also feel bad. Does anyone have any experience with anything like this Thank you.