Oomis, it’s true that when my in-laws were just nasty to me I didn’t have the courage to keep my children away from them, even when they interfered in my marriage. But when they saw that I ignored it they escalated by attacking my kids, and that’s when I found my backbone.
Unfortunately I know from a negative experience with my husband’s sister (long story short: she told my husband that I had no bitachon and did not love my children, because I would not quit the job that was supporting our family to live on government programs) that if I put anything in writing with these people they will twist it against me.
I did feel very guilty about limiting contact but after consulting both a rav and a therapist I was told that limited, carefully supervised phone calls and visits are the only type of contact they should have with my children.