Your sister's husband is your brother-in-law but English has no term for his sister's relationship to you. The dominant cultures of Europe and North America do not consider your sister's husband's sister to be related to you. The closest you can som is to call her you "brother-in-law's sister."
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.
brother in law
Mom
Sisters is a noun. Sisters is plural; the singular form is sister. Sister is a common noun, meaning the word can refer to any female sibling, not a specific one. Sister is a concrete noun also, meaning you can see a sister. Lastly, the possessive form of sisters is sisters'. The possessive form of sister is sister's. See the examples below: My only sister is my best friend. My two sisters are my best friends. My only sister's boyfriend gave her a ruby heart for Valentine's Day. Here the boyfriend belongs to my only sister. My two sisters' husbands are good men. Here husbands belong to my two sisters. MMP -- English teacher
Sister-in-Law
Your sister-in-law.
Many widows who have not remarreid and who are close to their sisters-in-law continue to consider them to be sisters-in-law. If the widow remarries, that can create a more complicated situation and they may be called "former sisters-in-law," or "the sister of my late husband."
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.
She is your sister-in-law.
My sister's son is my nephew.