Your mother's husband's sister is your mother's sister-in-law. Her sister-in-law's husband is not related to your mother, but is her husband's brother-in-law.
The most common ways of referring to him would be "my sister-in-law's husband" or "my husband's brother-in-law."
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
you call her your daughter's husband's sister
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 sister-in-law.
your brother-in-law :)
Your mother's sister's husband is your uncle, but his mother is not related to you.
You would call your husband's younger brother a brother-in-law.
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.