The mother of your sister's husband is your sister's mother-in-law. The English language has no specific term for a family relationship with your sister's mother-in-law, and you are not considered to be related to each other.
sisters in law
Your mother's sister is your aunt and her husband is your uncle. If he has no brothers, his sister-in-law must be either your mother or another one of your mother's sisters (your aunt)
She is your sister-in-law.
Brother-in-law.
your brother-in-law :)
Your sister-in-law.
Your mother's sister's husband is your uncle, but his mother is not related to you.
Your mother-in-law. Or, call her by her name.
You can call her by her first name or if your comfortable some son-in-laws call them mom.
Your mother's sister is your aunt. Her husband is your uncle.
Unless you are in a polygamous marriage, you are your husband's wife, so you would call her your mother.
Phillipp uncle