He is their Uncle and they are nieces and nephews.
Your sister's husband is your brother-in-law. Your brother-in-law's brother is not related to you and neither are his children. But he is your sister's brother-in-law and his children are your sister's nieces and nephews.
Your sister's husband is your children's uncle. They are his nieces and nephews.
Your husband's sister is your sister in-law.
Your sister's husband is your brother-in-law.
No, he is simply your sister in law's husband. He has no relationship with you. The reason she is called your sister-in-law and is because it is expected for a husband and wife to have children, and those children will join your family tree as they have some ancestors who are also your ancestors. Your sister-in-law's husband will never have ancestors that are also your ancestors.
None.Your daughter's husband is your son-in-law and his sister is your daughter's sister-in-law. But the English language does not consider you to be related to your daughter's husband's sister
Your husband's mother's sister is your husband's aunt. She is not realted to you.
The children of your husband's sister are your nieces or nephews, and you are their aunt, just as he is their uncle.
Your sister's daughter is your niece. Your niece's husband is not related to you.
Your sister-in-law's husband is never your brother-in-law. He can be your brother, when your sister-in-law has that relationship to you because she is married to your brother. You could also have a sister-in-law who is the sister of your spouse. But then her husband is not related to you.
She was the daughter of Zeus and Leto, and twin sister of Apollo. She had no husband or children, for she was a virgin goddess.
The English language has no term for a relationship between you and your husband's sister's husband. Most people would not consider her to be related to you. You could call her your sister-in-law's husband.