It's seems pretty clear to me. Are you the primary caretaker of a child or were you at one time the primary caretaker of a child? Yay! You are/were a mother.
You've never been the primary caretaker of a child?
You're not a mother.
I'm not a mother. I don't have a WAY to be a mother right now. I really want them to stop grinding it into my face. I know it's innocent and that there isn't malice behind it (unlike, clearly, your mother-in-law who in this one instance was a terrible person) but it hurts like someone has punched me in the stomach.