Your son and your mother's sister are in a "Great Nephew/Great Aunt" relationship.
Our cousins are our mothers sisters son :)
cousin
cousin
Your father's sister is your aunt and her son is your first cousin.
First cousin, once removed.
Your mums brothers sisters roommate flatmates sisters brothers mothers son.
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 second nephew
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.
Mother's grandson as regards her daughter is either her (daughter's) son or her (daughter's) nephew.