Why do parents choose one kid over the other?

My fiancee parents let his older brother live in there house for free. The older son punched the dad in the face and the dad moved out of his own home. He lived off my fiancee for a year and then finally got a job and moved out. The mother then moved in with the dad and they let the older son and his scummy wife live there for free . It's been 2 years now and every time they come to our house thet talk crap about how the older son owns them money and how his wife never cleans the house and it disappoints them. This makes my fiancee fell like they don't even care about him. Oh and when they come over they are always drunk off their a$$es. My fiancee is sick of it and would just like his parents to stop coming over. What is a nice way to tell them to stop coming over and to deal with their older son & his wife. One more thing the older son's wife got the son busted with an ounce of pot that was for her,they got into a fight and he called the cops because she hit him and he had a oz on him so he put it in the trash and she told the cops. Now he's on probation and she still smokes pot in the house. This is one efffed up situation and I hate all of them. Please help we can't take much more of the BS. I want to be as respectful as possible . His mom is a loony toon and had a stroke a year ago, and drinks like a fish, cuz the scummy daughter-n-l aw told her that hops in good for stroke patients.So I don't want her to stroke out, but I want them to know how we feel. Please be kind! This is just one of the incidents that has happened there is so much more I could tell but this is the latest one.