Second cousins.
Your sister would be her great aunt and she would be your sisters great niece.
The grandchild of your grandmother's first cousin is your third cousin. The grandchild of your grandmother's second cousin is your fourth cousin. The grandchild of your grandmother's third cousin is your fifth cousin.
Your daughter and your sister's child are first cousins. Your daughter and your sister's grandchild are first cousins, once removed.
Your great uncle's grandchild is your second cousin.
second cousin
Your great grandmother's sister is your great great aunt. Her great grandchild is your third cousin.
He would be his great great great uncle
First cousins once removed.
second cousins twice removed.
Well that would depend on if you were the biological grandfather. If so then the grandchild is also your grandchild. If the child was born to a child of your ex-wife and not fathered by you then they are no relation to you and you would call them by their name.
Your great grandfather's sister is your great great aunt. Her grandchild is your parent's second cousin and your second cousin, once removed.
You and your first cousin's grandson are first cousins, twice removed, to each other.