Your great grandfather's sister is your great great aunt. You are her great great niece or great great nephew, depending on your sex.
She will be your aunty
Your great grandfather's half-sister is your great great aunt, the same as his full-sister is.
Two people whose grandfathers were brothers are second cousins to each other.
Your grandfather's great nephew is your second cousin.
She would be your great grandmother
Your grandfather's cousin is your cousin twice removed. Her husband has no direct relationship to you.
Your grandfather's sister is your great aunt.
He would be your great-great uncle.
Yes, your grandfather's granddaughter could be your sister, making her son your nephew.
He had a good relationship with his sister
The possessive form of the plural noun grandfathers is grandfathers'.example: Both of my grandfathers' names are George.