Your grandfather's brother is your great uncle. Your great uncle's daughter's daughter (his granddaughter) is your second cousin.
Your grandfather's brother's granddaughter is your second cousin.
He would be your great-great uncle.
If you really need a relationship term, use granddaughter-in-law. Most people would merely say "my grandson's wife."
Your aunt is great great aunt to your son's daughter (your granddaughter). Your granddaughter is your aunt's great great niece.
Your son and your sisters child are first cousins. The granddaughter is out of synch by a generation. That will make them first cousins once removed.
Your daughter's son is your grandson and you are his grandparent.
Your brothers great granddaughter is your great great niece.
Two people whose grandfathers were brothers are second cousins to each other.
He would be your great-great uncle.
yes
Yes, your grandfather's granddaughter could be your sister, making her son your nephew.
You would be 2nd cousins, assuming that the brother's wife's granddaughter is also the brother's granddaughter. If the granddaughter is not the granddaughter of the brother, there is no relationship.
She will be your aunty
Your sister is her great grandaunt.
Your granddaughter is your sister's great niece. Your sister is your granddaughter's great aunt.
Your granddaughter and your niece's son are second cousins.
Your sisters grandson would be your grandnephew.
Your granddaughter could be your son's daughter, or your son's niece.