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 second nephew
cousin
Your father's sister is your aunt and her son is your first cousin.
If your niece's is by your sister but the brother is by your sisters new husband & another woman there would be no blood relation. If not then he's the nephew
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
Well, it depends how close you are to your husbands family. If you really want to know they are cousins-in-law.
Your sister's son is her grandson and your great nephew. Some use the term "grand nephew."
Your sister's son is your nephew His sons are your grandnephews, and his daughters are your grandnieces.
Your son would be the uncle of his brother's son.
Your sister's son is her grandson and your great nephew. Some use the term "grand nephew."
If your half-sister had a son that would be a half-nephew to you
You would be a great uncle, so they would be your great nephew