Not of their own; this is one of the reasons they took young John in.
You are their aunt or uncle
In most families they would call you aunt or uncle. It would technically be great aunt or great uncle.
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.
aunt, uncle, grandpa, grandma, cousin if old enough...
you are their gran-uncle or gran-aunt depending on your gender. they are your gran nieces and gran nephews depending on their gender.
In English there are no masculine or feminine forms. English uses gender specific nouns for male or female.The gender specific noun for the sister of your parent is aunt.The gender specific noun for the brother of your parent is uncle.
James has two godparents who are (of course) Hermione Granger and Ron Weasley, the kids' aunt and uncle
Barry Sanders was one of 10 kids. He had 2 brothers Boyd and Byron and 8 sisters
Well, they are related but a name has not been given to that yet! :)
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."
No his sister Gemma has no kids
Your grandfather's brothers would be the uncles of your parent. That would make them your great uncle.