Your cousin's wife is your cousin-in-law because you are only related by marriage and not by blood. The "-in-law" suffix can describe the relationship between you and the spouse of any of your blood relatives.
The "-in-law" suffix is only applicable to people who marry your relatives and not their families. Though your cousin's wife is known as your cousin-in-law, there is no generally accepted term for referring to your cousin-in-law's mother. It is easiest to use names or descriptions for these people depending on the context. You could say "Helen's mother" to someone who knows your cousin's wife, or you could use "my cousin's wife's mother" to explain the relationship to a stranger.