yes, the french word is ambiguous.
Usually, is means mother in law.
But the same word can be used for a stepmother.
So, if you are talking about your stepmother, you may have to give more detail to remove the ambiguity.
Yes, indeed.
I remember having used the phrase le mari de ma mère to clearly talk about my step-father when I got married, as I then had... two beaux-pères!!