cousin once removed
Your grandfather's brother's grandson is your great uncle's grandson. The grandchildren of great aunts and great uncles are your second cousins.
Your grandson and your brother's grandson are second cousins to each other.
Your niece's grandson is are your great-grandnephew.
They are your Uncles and or Aunts.
not to be complicated , but if you have an uncle--> he has a nephew--> as you can see that's not complicated . And if you ask what is a uncles NEPHEW related to an uncles grandfather. in what way ....i would say the nephew is is the great gandfathers great grandson.
Your sister would be your grandson's great aunt.
Your first cousin's grandson son is your grandson's third cousin. Your second cousin's grandson is your grandson's fourth cousin. Etc., etc.
Your sister is your grandson's great aunt. He is her great nephew.
Your great aunt's grandson is your second cousin.
great uncle
You Aunt is you grandson's Great-Great Aunt. Your grandson is your Aunt's Great-Great Nephew.
Your grandson is your brother-in-law's great nephew, and your brother-in-law is your grandson's great uncle.