My tenant seems to have moved out and moved his sister in

[Ronda R.]

I got a call from my 3rd floor tenant (triplex) that one of my tenants seems to have moved his girlfriend in and I have been suspicious of it for a while. We have crossed paths a few times and she won't even return my greetings but gives me the evil eye. I have been weighing my options but really don't want her on the lease because I really don't want to deal with her. I like him and he has been paying rent on time. He is respectful, she looks at me like she wants to kill me. I had a friend with me one time she was leaving the building and she said, "Wow that girl is scary mean." So I don't really want to give her legal right to be there. 

Now I just got a call that the bottom floor tenant seems to have moved out and let his sister and boyfriend move in. I don't have proof of either. What would you do? I want to get on the phone right away but want to get advice first.