opinions?

my and my fiancé have been together for 6 years as of Wednesday. Last year his mother and niece had nowhere to go so I allow them to move in. I had a few rules but she refuses to follow them because she says she does not follow rules from a 21 year old. For example I have the rule of no smoking anywhere in the house because of my 2 children. She refuses to follow that rule. She was supposed to move with her taxes and now she is wanting to stay here longer. it has caused relationship issues between me and my fiancé and he is constantly telling me im immature now. His niece is 5 and my son is 3. I know they are going to fight and all that but it gets to the point it goes way to far and the 5 year old starts hitting my son in the face and biting. If I get on to her his mother constantly yells and tells me I have no say so cause she has custody of her. But if my son hits her granddaughter she is the first to pop his but or put him in time out. My fiancé tells me im not allowed to put her out so I have no options. Any opinions on what to do with this situation???