M.G.
I am sorry for your loss.
I went through what you are describing. It's not your SIL that is the problem. Well, she is a problem, but she's not yours. Your husband does not have boundaries. Your family (you/he/kids) needs boundaries. You need to agree on this and enforce them together. Rather, he enforces them with his family members, and you support him.
My husband never took his frustration out on me or blamed me. It was very clear his family was being too demanding of him and not respectful.
I wouldn't fixate on your SIL. This is about how your husband is treating you. Tell him it's not acceptable. It doesn't really matter what the reason is - he shouldn't be taking his stress out on you. My husband went to some counselling sessions on his own to deal with the issues in his family. They'd been going on since he was a kid.
Parents coming to their son is not right. That's the problem we had (still have). Not while they are still able to look after themselves. My inlaws go to my husband about his brother (still lives at home). My husband learned, through counselling, to care but not get involved.
My husband didn't like his family coming to him. Sounds like your husband might like it. That's a whole other kind of problem - codependency perhaps.
Your husband and you would really benefit from counselling. You have boundaries about what you will accept from him (and what you won't - insults, etc.) and he needs them with his parents and sister, and as a family you and he need to decide what you will and will not let affect your marriage.
If he does not agree to go to counselling, then you go and get help with how to best deal with this. You have enough on your plate. Good luck