Call him whatever his name is.
If your uncle is the brother of one of your parents, then his wife's sister and her son are not related to you.
If your uncle is the husband of one of your parents' sisters, then his wife's sister may be:
Your uncle's wife is your aunt. Her sister is not related to you and neither is the sister's son.
Probably your niece-in-law.
Your uncle's son is your first cousin. Your uncles grandson (son of his son) is your first cousin, once removed.
We would infact, cll our sisters son your nephew and they would call you either auntie or uncle depending on your sex.
cousin brother
Your uncle's wife is your aunt. Her sister is not related to you and neither is the sister's son.
Probably your niece-in-law.
Your uncle's son would be your cousin. Your parents' brothers and sisters are your aunts and uncles. All of their kids, regardless of gender, are your cousins.
Your cousin in law
uncles
Your uncle's son is your first cousin. Your uncles grandson (son of his son) is your first cousin, once removed.
If your sister had a son, then he is your nephew.
nephew
Your cousin.
Our cousins are our mothers sisters son :)
We would infact, cll our sisters son your nephew and they would call you either auntie or uncle depending on your sex.
My sister's son is my nephew.