He doesn't talk when at your sister's house? Was he anti social or socially awkward when you met him?
Pp are dismissing cultural differences.
I'm of African origin, but it strikes me your DH had a very poor life in comparison, as Christmas is celebrated Africa....even in the villages where poverty prevails. Although this likely varies from one country to another.
So if he was so poor growing up...it might account for a lack of education and awareness of social skills/etiquette. Most people would understand it's poor form to pull out from the invite so late, with no good reason.
Is his friend from his own country?
It could also be he just can't relate to your sister and her BF. Is he a professional? Does he speak English well?
These are reasons I've known people to not socialise with spouses friends...because they feel unable to converse on the same level...or they may find it difficult to understand them...due to a strong accent.
My friends husband once told me our conversation goes over his head. The difference is he's a very socially man.... and would interject and ask for clarification on what I thought was an obvious thing.
Not everyone is that courageous. I know I wouldn't be. I've often quickly googled a word I didn't understand...ad I'd be embarrassed to ask.
Ultimately you know if you can carry on living with him as things stand. By not going to your sisters he gets away with it.