When someone's nephew has a child, that child is considered a grandniece or a grandnephew to the nephew's aunt or uncle. The terms great-niece and great-nephew are also appropriate for this relationship. Children of the nephew can refer to the nephew's aunts or uncles as great aunts/uncles or grandaunts/uncles.
If someone's grandniece or grandnephew has a child, the child becomes that person's great-grandniece or great-grandnephew. He or she may also be referred to as a great-great niece or a great-great nephew. The great-great niece or nephew can call the original family member a great-great aunt or uncle or refer to the person as a great-grandaunt/uncle.