What to call father-in-law's wife?

I would find out what they want you to call her.

Since she's your FH's step mother, she would be your step mother in law

If your introducing them to someone I go with this is my father and his wife (or in your case father in law and his wife) or you could say this is Joe and Jill, Fred's dad and step mom.

If you are talking to them they get called their names. Or Mr and Mrs if they are overly formal.

I would have w talk with them and FH and see what everyone would like to be called does FH call her mom or any variation of it?

FH has a step mother and a step father. I also have a step father. We just call them what we usually call them (by their first names). It depends on the relationships that have already been established.

Depends on your relationship with her, if it were me, I'd just ask her

I call them by their names. But then my husband will always say " Mary my father's wife, or John my mother's husband" and he has a good relationship with both.. Don't over think it

She would be your step mother in law

Technically speaking she would be your step mother in law.

Step mother in law. I call FH's parents by their first names.