Your sister's child and your grandson are first cousins once removed.
Your sisters grandson would be your grandnephew.
Your granddaughter and your sister's grandson are second cousins.
Your niece's child and your grandson's child (your great grandchild) are second cousins, once removed, to each other.
If your daughter-in-law's grandson is a child from a marriage before she married your son, or after their marriage ended, her grandson is not related to you. If he is a child of the marriage between your son and your daughter-in-law, then he is your great grandson.
Your sister's great grandson is your great great nephew. You are his great great uncle, or great great aunt.
Your grandson is the great-nephew of your brother.
Your niece's child and your grandson's child (your great grandchild) are second cousins, once removed, to each other.
Your sister's child is your nephew if it is a boy, or your niece if it is a girl.
if you are : 1. male , you are known to be as the child's uncle. and 2.if female , aunt
Your sister's son is your nephew and you are his aunt (if you are female) or his uncle (if you are male.
Sequence of relatonship:Your uncle is a sibling of one of your parentsHis child is your 1st cousinHis grandson is your 1st cousin once removedHis great grandson is your 1st cousin twice removed
Your great great grandmother's sister's great great grandson is your third cousin, twice removed.