I don’t think this is unusual. As parents get elderly they don’t stand up to their children and get stuck in situations. I know of two similar sad situations.
Discontinue to provide internet service and get your own cell phone. Unhook every possible landline you can... And that computer gaming fool will probably leave for greener pastures.
Oh, and don’t forget to start cooking from scratch. Everything. no cans no boxes just raw ingredients and the idiot in the spare bedroom won’t even know how to graze.