Your sister-in-law's daughter-in-law is not related to you but the daughter-in-law's daughter is your great niece. You are her great aunt (if you are a woman) or her great uncle (if you are a man).
Because her daughter will already be like a miniature mother, with skills like that. But a mother likes to fuss over her son, cook tea, do his ironing etc but a daughter in law takes these roles from...
If (as a man) you married your sister-in-law's sister then her mother is your mother-in-law. If (as a woman) you married your sister-in-law's brother then her mother is your mother-in-law. If your...