I need to vent. So my wife's sister is almost 40yo and has an 18yo son. Her and her son live with my…

I need to vent. So my wife's sister is almost 40yo and has an 18yo son. Her and her son live with my MIL. Over the years I have taken him to Dr appointments, parent/teacher meetings, driven him to and from school bc where we live high schoolers don't have bus transportation and it gets really cold in the winter. Well, his mom has the nerve to talk shit about me to any and everyone, all while she doesn't even really know​ me. She has never really acted like a mom let alone an adult. One of the first things she ever told me was how she used to be an escort. Needless to say her opinions of me don't really bother me. What bothers me, is everyone in the family listening to her instead of getting to know me. Now here I am waiting on my wife to get up so we can all get ready and go over to my MIL house bc yet again, they want me to be the parent to the 18yo. He has been following in his mother's footsteps and expecting everyone to cater to him. His mom recently quit her job and decided to spend a month in Arizona. Meanwhile, her son has dropped out of college, using my MIL card to pay for endless Uber rides, eats all her food, has vast amount of friends over at all times, doesn't help out around the house, doesn't pay any bills, he does have a job, but he still expects to be handed a free ride. Now I am expected to go over there and tell him how things are going to be from now on or he can get out. They even want me to type up a contract for him to sign. Like y'all either need to like me and accept our relationship, or just stop and leave me alone when it comes to family issues. You can't have it both ways.