Putting on clothes for a visitor, even an unannounced visitor, is a pretty normal expectation. If the son-in-law is trying to communicate that the LW is unwelcome, either because she drops by unannounced or because she stays too long, then doing so by remaining undressed is passive aggressive bs - not an acceptable substitute for a real conversation about privacy and boundaries. (Although clearly not all her visits are unannounced; the SIL should have expected she would return his kids to him.) "You're killing me in my own house" is also ridiculously over-dramatic. Personally, I think the SIL is a jerk. The LW should hold fast to her entirely reasonably request that he wear clothes when she's there and not let his petulance get to her.