Your great grandmother's brother's great grandson is your third cousin.
1st cousin 2x removed
You are third cousins.
Your great great grandmother's sister's great great grandson is your third cousin, twice removed.
First cousins twice removed.
Your grandson is the great-nephew of your brother.
My ancestors
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.
Your sister's great grandson is your great great nephew. You are his great great uncle, or great great aunt.