It's simple, he doesn't like your sister and/or her husband. Whatever the reason for this, being rude to a guest when they're on your turf/at your mercy is in bad taste.
The excuses being made for him/men in general are a tad surprising. Being a considerate host, treating guests with kindness is not gender-specific behavior. Men are not clueless gnats. Not being "social" is not an excuse for locking yourself in your room to avoid guests, let alone relatives living in your house. It's becoming of a sulking teenager, not the head of a household.
I'm not excusing it. Simply trying to explain what I have observed.
Sure, my intention wasn't to single anyone out. It was more a general remark about society. I think it's sensible to give people the benefit of the doubt. My own reflex is to attribute bumbling but benign social ineptitude to men, but I'll call a woman in the same position petty. Given OP knows her spouse better than anyone, if she says he's hospitable with everyone but the SIL I'm disinclined to attribute a social blind spot. This thread's triggered memories of a looney aunt who happily unleashes her passive-aggression on house guests. [cue traumatic flashback]