Your cousin’s wife is your cousin-in-law because you are not related by blood but rather by marriage. The suffix “-in-law” can be used to express your relationship with the spouse of any of your blood relatives.
The suffix “-in-law” only applies to those who marry your relatives, not their family. There is no commonly accepted phrase for referring to the mother of your cousin-in-law, despite the fact that your cousin’s wife is known as your cousin-in-law. Depending on the circumstances, it is easiest to use names or descriptions for these individuals. You may say “Helen’s mother” to someone who is familiar with your cousin’s wife, or you could say “my cousin’s wife’s mother” to a stranger.
