You are not related by blood to your cousin’s wife, so she is your cousin-in-law. The word “-in-law” can be used to describe your relationship with the spouse of any blood relative.
The suffix “-in-law” is only used for people who marry your family members, not their families. Your cousin’s wife is called your cousin-in-law, but there is no standard way to call your cousin-in-mother law’s by that name.
Depending on the situation, it is easiest to call these people by their names or describe them. You could say “Helen’s mother” to someone who knows your cousin’s wife, or you could say “my cousin’s wife’s mother” to a stranger.