You are not responsible for the tension or division in your husband's family. Even if the relationship with your mother-in-law were less fraught, it would be primarily your husband's job, not yours, to plan get-togethers or draw harder boundaries (with input from you, of course).
I also think she is a toxic person, and a close relationship with her is not in my kids' best interest. Setting boundaries to protect your children from negative influences is essential.
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