Your son and your mother's sister are in a "Great Nephew/Great Aunt" relationship.
Second cousins.
Your mothers sister is your Aunt
Your sister-in-law's son is your nephew.
Your husband's sister's son is your nephew, just as your sister's son is your nephew.
The sister's son would be the nephew of her brother.
Aunt Aunty Auntie
Your nephew.
If your sister-in-law is the sister of your spouse, her son is not related to your sister's son. If your sister-in-law is the wife of your brother, her son and your sister's son are first cousins.
Your sister's son is your nephew. You are his aunt, if you are female, or his uncle, if you are male.
Your sister's son is your nephew. You are his aunt if you are a woman, or his uncle if you are a man.
You are the great aunt/uncle of your sister's son's daughter, and your sister's son's daughter is your great niece.
Our cousins are our mothers sisters son :)
If your half-sister had a son that would be a half-nephew to you