Hi Musing

Welcome to our community.

It sounds like your SIL has brought a lot of conflict into your life, and you are kind of at your wits end and need to self-protect, so you are seeking a way to have less contact with her, but without feeling her "wrath".
This site has so much info on "tools" we can use to manage the high conflict personality in our life. Have you had a chance to find this section yet? I have really found the info helpful, and put it to use in my daily life. When I was new here, I started with the section "how to get the most out of this website". Kinda life changing really.
As for your situation, does SIL live close to you? How often is there contact with her? What does that contact look like?
What is your husband's relationship with his sister like?