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.
Your husband's sister's kids are referred to as your nieces and nephews by marriage. More specifically, you can call them your "nieces" if they are girls and "nephews" if they are boys.
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.
Your mother's sister's husband is your uncle, but his mother is not related to you.
Brother-in-law.
your brother-in-law :)
Your sister-in-law.
You would call your husband's younger brother a brother-in-law.
Your mother-in-law. Or, call her by her name.