Your aunt's great grandson is your first cousin twice 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 great great grandmother's sister's great great grandson is your third cousin, twice removed.
it's like your 3rd cousin
Your brother is your grandson's great uncle, and your grandson is your borther's great nephew. Some people would also say grand nephew and grand uncle.
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.
Your sister is your grandson's great aunt. He is her great nephew.
Your great aunt's grandson is your second cousin.
Your niece's grandson is are your great-grandnephew.
First cousins, twice removed. Aunts daughter is your cousin. Aunts daughters daughter is cousin once removed Aunts daughters daughters daughter is cousin twice removed.
Your great grandmother's brother's great grandson is your third cousin.