a nephew's kid to his aunt's kids would be their cousin.
you are their gran-uncle or gran-aunt depending on your gender. they are your gran nieces and gran nephews depending on their gender.
Well, they are related but a name has not been given to that yet! :)
Your grandfather's brothers would be the uncles of your parent. That would make them your great uncle.
Your sister's husband is your children's uncle. They are his nieces and nephews.
Your uncle/aunt's son/daughter is an adult and you are still small, your uncle/aunt's daughter had a child, and that child will call you uncle. Uncles/aunts are your dad/mom's siblings.
Your immediate family are your parents, siblings, spouse, and kids. Every one else, (your grandparents, aunts, uncles, cousins, nephews, nieces, etc) are your relatives.
Your aunt's son is your first cousin, once removed. Your great aunt's son and one of your parents are first cousins. You are one generation removed from them so you and your great aunt's son are first cousins, once removed.
Call the great nephew Joe and the great aunt Pamela. If Pamela is the sister of Joe's grandmother or grandfather, then Joe and Pamela are "blood relatives." However, Pamela could be the wife of a brother of one of Joe's grandmother's or grandfathers and still be Joe's great aunt. In that case, Pam and Joe are not "blood relatives."
The children of your husband's siblings are your nieces or nephews. Their children are your great-nieces or great-nephews.
They are your nieces and nephews.
Your dad's nephew is your first cousin, his children are your first cousins once removed.
Your aunt is the sister of either your father or your mother. If your aunt is your father's sister, then your father's cousin is also her cousin. If your aunt is your mother's sister, then she is not related to your father's cousin.