I'd use their name. There is no official name for it. You do not share a common ancestor, so you are not related.
Your sister's husband is your father's son-in-law.
Yes, for a nephew is sister's son, brother's son, wife's brother's son, wife's sister's son, husband's brother's son, husband's sister's son
Your sister's son is not related to your husband's sister's son, because they share no common ancestor.
No. Your sister's husband is your brother-in-law, but his sister (and her son) are not related to you. However, that son is your sister's nephew.
Your husband's sister's son is your nephew, just as your sister's son is your nephew.
Your dad's sister is your aunt. Her husband is your uncle. His sister is not related to you, and neither is her son.
Your father's sister is your aunt. Her brother-in-law is either the brother of her husband or the husband of her sister. If you are talking about your aunt's brother-in-law's son, the English language has no terms for a relationship between you and him. We would say he is not related to you. If you are talking about the son of the husband of your aunt's sister, since your aunt's sister is also your father's sister, and her husband's son is her son, the son is your first cousin. If the son is the result of a previous marriage, then he is not related to you unless your aunt formally adopted him, in which case he would be your first cousin.
No, you have no ancestry in common with the son of your aunt's husband's sister, so you are not related to him..
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.
Your father's sister is your aunt. Her brother-in-law is either the brother of her husband or the husband of her sister. If you are talking about your aunt's brother-in-law's son, the English language has no terms for a relationship between you and him. We would say he is not related to you. If you are talking about the son of the husband of your aunt's sister, since your aunt's sister is also your father's sister, and her husband's son is her son, the son is your first cousin. If the son is the result of a previous marriage, then he is not related to you unless your aunt formally adopted him, in which case he would be your first cousin.
Your husband's sister is your sister-in-law. Her husband is your husband's brother-in-law. But your sister-in-law's husband is not related to you.
Your husband's sister is your sister-in-law. Her husband is your husband's brother-in-law. But your sister-in-law's husband is not related to you.