Stupid sister in law

It was my husband's birthday yesterday. He used to be very close with his younger sister, even a few years into our relationship. But then I started to see how badly he was treated by his mom and his sister, then as his sister got older and bitchier (late teens/ early 20s) that's where things really went wrong but somehow I got the blame.

His mom and sister are both classic narcissists and I think it drove them crazy that he had begun to see that they were actually quite awful to him (and now many years later, they're awful to each other too).

Anyway, there was an incident going on three years ago where his younger sister wanted to come stay with us in their mom's home which we pay full rent for, so don't have to let anyone we don't want stay (she lives in another country now) . We said no because of how I have been treated in the past by them as well as the fact that they tried to ruin our wedding then didn't even come. Did his sister not throw the biggest fit and basically told us we should have gotten over that by now, it's been years ago, do we turn my family (who are amazing) down if they want a place to stay, etc.

And she hasn't spoken to him since. Three birthdays she hasn't bothered to put her childish pride aside. Three years and she still thinks she was in the right. He had a fight with his mom shortly after and even she's somewhat gotten over herself (although their relationship hasn't been the same).

I know some of you may think, well why doesn't he phone her. She blocked his numbers and has probably also changed phone numbers too so it's not that easy, and none of them are on social media so he has no way of contacting her.