Forgive the family member in advance, the parent that you hang out and usually have fun with. Maybe they really didn't know the son started moving in, i'm gonna call it "started moving in" - because he only brought enough stuff into the house, that he was able to clear out in apparently one or two trips, the same day you called him and expressed displeasure in his being there.
So its not like he changed over the utiities and then didn't pay, or parked 3 broken down jalopy cars on the property, or busted up the house and stole the copper pipes from the walls. Or even cooked more than 1 or 2 meals in the kitchen.
MAybe he had been living with a girlfriend before this and they happened to get in a big fight -the same week that coincidentaly his parents asked him to stop by your house and make sure the electricity was off and the faucets weren' t leaking. Who knows ? -- In my made-up example his parents didn't know he had stayed there 1 night yet, but thought he was @ girlfriends, but they did give him the key, so they felt sheepish when answering you.
--You already expressed the displeasure, they know, maybe they are giving you time to "cool off" before they call.--
Do the final clean of the house, list it in the MLS, then call relative, go out to lunch to celebrate getting the house on the way out of your life, and ask how their son is doing , while trying to come from a place where you care about their problems , while minimizing the hurt feelings that their son, -*almost *- took advantage of the shelter that your parents old home seemed to offer.
Youch --- i just read the 2 replies that landed before mine - How about the Orig Poster assumes for a moment that the parents didn't on purpose move Junior into the house. Its not like the other relative [ that called with the warning] said " I saw your Aunty/uncle with a moving truck carrying mattresses and Jr's comic book collection into your parents old house...." - Be mad at the wayward 20 something yr old for being tempted to squat, but i'm not sure how we see that aunty/uncle purposely aided and abetted Jr's slumming adventure.
Quote from orig post *** "I normally see this family member, go to lunch, go out and have fun" *
- I suppose they were just waiting to spring this malfeasance on you the whole time they were pretending to have fun with you over the years IslndFI girl . Sarcasm.