Your uncle's father is also the father of one of your parents and thus your grandfather. You are his grandchild.
If your grandfather has only one grand child, that person is you.
So your uncle's father's only grandchild is you, yourself.
The grandson of your great uncle is your second cousin.
or 3rd cousin.
You grandson is great great nephew to your uncle. You uncle is great great uncle to your grandson
You grandson is great great nephew to your uncle. You uncle is great great uncle to your grandson
You and your great uncle's grandson are second cousins.
Your sister's son' son is her grandson. Your sister's grandson is your great nephew. You are his great uncle or great aunt.
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 sister's son' son is her grandson. Your sister's grandson is your great nephew. You are his great uncle or great aunt.
Your great great uncle's great great grandson is your third cousin once removed.
Your daughter and your uncle's great great grandson are second cousins, three times removed.
Great great aunt/uncle. To your niece, you are the aunt/uncle, to her child you would be the great aunt/uncle. So you would then be the great great aunt/uncle of her child's child (her grandson)
Your brother is a great great uncle to your great grandson. Your great grandson is your brother's great great nephew.
You are a great great Aunt (or Uncle) to the grandson of your Nephew.
Your great great great uncle was the brother of one of your great great grandparents. His grandson is the second cousin of your grandparent who is descended from that great great grandparent. Therefore his grandson is your second cousin, twice removed.