I just think this should not all be your sisters responsibility and if she and her husband are not on the same page in relation to Sam that can and will cause serious problems in their relationship.
IMO Sam needs some help and direction. Being different may not necessarily be an issue but some of the anti social behaviour can be a serious concern.
Am assuming your sister is not too far from your age so would also say that t that (our) age you take on lot of baggage. Not trying to equate a human being as baggage but when you take on him you take on his son.
Nothing wrong with being anti social per say, or watching world news, but his panic & fear of being left alone and anxiety are issues.
One parent, let alone a step parent, can't deal with these issues on their own. They, as a couple, deal with this and seek professional help, or they don't.
If the Father refuses to deal with these issues to help his son it's up to your sister whether she stays or goes.
Not disagreeing with any of above, But I think, if you'll allow me to generalise a little Men & women have different needs from a partner & household chores, cleanliness, putting manners on someone is only a part of it. If it was just those issues, hell yeah I'd agree with Serene but women also have needs & wants & they can differ.
RE: Guys vs Girls
I don't know495