My husbands relatives live with us and are disrespectful.?

My husbands sister and boyfriend have lived with us for 2 years. We said that they could stay with us until they found a place of there own. They have a child that colors on my walls and hits me. I don't spank him or tell him no. I respect what his parents ask and don't do anything. I've asked them nicely to teach him not to do these things. They don't care or respect my wish. The boyfriend breaks things and avoids talking about them and paying. They thrash my house and don't clean up after themselves. They only pay 350.00 rent that includes everything internet t.v. water electricity. My husband loves his sister and doesn't want them to go on a bad note. How do we get them to leave.