How do you get a non-paying non-relative to move if she will not leave voluntarily?

Bare with me because this gets complicated... My mom and step-dad (Pete) live with my step-dad's ex-wife's mother. Pete and his ex-wife bought the house from her mother (Judy) and started building a house in the lot next door. Then Pete found out his wife was cheating so they got a divorce. At the same time, Judy and her daughter (Pete's ex-wife) got in this huge argument and were not talking to each other. Pete is a really nice guy (and kind of a push over) so he told Judy she could stay with him. The ex-wife got the new house in the divorce, and Pete got what used to be Judy's house. He had previously paid off the mortgage. Then Pete and my mom got married. Judy is completely disrespectful of my mother, and is now on good terms with her daughter. My mom and Pete's marriage is suffering because Judy is living with them. Judy refuses to move, stating that it is her house and she doesn't have to move. She is no longer the house owner, and my mom and Pete have made tons of improvements on the house and even have a new mortgage. How do they legally make her move, if she will not leave voluntarily? Any help would be greatly appreciated.