The great grand father would be the great grand sire, I'm not sure what the grandson would be. I'm sorry.
the answer is in www.haishoe.com
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.
You have no socially named relationship to your step-great-grandson-in-law's father. If you like, you might call him "son" or "Mr. ...." or simply call him by his first name.
Your brother's grandson is your great nephew.
Your brother-in-law is your spouse's brother. His grandson s your spouce's great nephew. (Some people say grand nephew). That makes him your great nephew, too. Your brother-in-law could also be the husband of your sister. Then your brother-in-law's grandson is your sister's grandson and your great nephew.
Your sister's son' son is her grandson. Your sister's grandson is your great nephew. You are his great uncle or great aunt.
The son of your grandson is your great grandson. You are his great grandmother or great grandfather.
Your father's great grandson could be your grandson, or your great nephew.
Your brother is a great great uncle to your great grandson. Your great grandson is your brother's great great nephew.
You grandson is great great nephew to your uncle. You uncle is great great uncle to your grandson
You Aunt is you grandson's Great-Great Aunt. Your grandson is your Aunt's Great-Great Nephew.
You grandson is great great nephew to your uncle. You uncle is great great uncle to your grandson