What if my boyfriend's sister steals?

I'm not sure what to do I didn't want my boyfriends sister to come over to our place because she has a reputation for stealing. Not just from stores but from family and friends. This weekend we had a b-day party for my boyfriends daughter and his step sister called and asked if she could come, he told her she could and then I called her back and told her no it's not a good idea theres already too many people and I have a small apartment. She got mad and felt like I was keeping her from her brother and who am I to say she can't come over, it's his house too. The thing is everything in our apartment is practically mine from my family. Now she coming over this weekend just her and my boyfriends other sister. She's complained about me to everyone and I've never met this other sister of his yet. I'm worried his stealing step sister is making me sound like a b*tch. I don't want either of them to get the wrong impression of me, should I just come out and say it, should I tell her I don't want her stealing from me or her brother and that if she does she'll never be welcome here again, or should I keep my mouth shut and my eyes open and hope nothing goes missing?

well if he steals that isnt good but your sister is her own self cant make her dump him because u think he steals
right

annikaxxxoo