He's called your uncle becasue your mother's sisters are your aunts and the people married to them are your uncles (assuming that they're men).
Mom
Our cousins are our mothers sisters son :)
you call her your daughter's husband's sister
Your brother would call your husbands sister by her first name. Or, "my (sibling's) sister in law.
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 could call them as a brother/sister or as a cousin... If you're asking for yourself then good luck with that family.
brother in law
cousin
Sister-in-Law
In Hindi, you would call your mother's sister "mausi" or "masi".
Your sister-in-law.
Let's see if we can unravel this. Your sister's husband's father's brother is your sister's husband's uncle. The uncle's daughter is your sister's husband's cousin. There is no relationship to your sister, unless your sister wants to call her a cousin-in-law. To break it down to its simplest terms: She is your brother-in-law's cousin and no relationship to you.