Your grandfather's brother's grandson is your great uncle's grandson. The grandchildren of great aunts and great uncles are your second cousins.
You Aunt is you grandson's Great-Great Aunt. Your grandson is your Aunt's Great-Great Nephew.
Your brother is a great great uncle to your great grandson. Your great grandson is your brother's great great nephew.
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 great aunt's grandson is your second cousin.
Your sister is your grandson's great aunt. He is her great nephew.
Your niece's grandson is are your great-grandnephew.
Your great aunt's great grandson is your second cousin, once removed.
Your great grandmother's brother's great grandson is your third cousin.
Your uncle is your child's great uncle and your grandchild's great great uncle. Your grandson is your uncles great great nephew.
Your grandson is your brother-in-law's great nephew, and your brother-in-law is your grandson's great uncle.
Your sister would be your grandson's great aunt.