Your grandfather's brother is your great uncle. His daughter (or his son) is your parent's first cousin and your first cousin, once removed.
Your second cousin once removed.
Are you asking what you call your brothers daughter? Well if you are your brothers daughter is called your niece.
A niece
A niece
Niece
A cousin.
Your niece.
natini
She will be your aunty
Your nephew?
Its would be your neice
Niece
Two people whose grandfathers were brothers are second cousins to each other.