Are you married or in a civil partnership? If not then she's not actually your mother in law anyway.
She’s not your mother in law if you’re not legally married to her son. I’ve been legally married for 8 years now and I refer to my husband’s mom as “Monster in law” I also have other names for her but only say them when I’m highly annoyed with her bullshit. In person I call her by her first name and refuse to use her nickname like the rest of the family.
LOVE this energy. Also subconsciously it’s what I do, I would never refer to her as MIL irl but never thought of it before