D.P.
I think your husband should tell his mom that he will not have further discussions about his sister's attitude or divorce. Will he do that and will she drop it then? Those are the questions.
Venting is O. thing but this seems like obsession.
Your SIL may still be angry and hurt but she has no right poisoning her kid's minds like that. That IS being mean.
If he won't be that direct, what I would do (and have done) is whenever the taboo subject(s) are brought up, say "I have to go--talk to you later. Bye." Click. I would do the same thing when the SIL gets on her rants as well.