Often in a large family, the younger children will become aunts and uncles while still kids themselves. Forrest, our youngest boy, became an uncle at age 10. As my grandchildren grew, they of course bonded with Forrest who was closest to their age and liked to play with them. He was their favorite, (altho they never really called him Uncle Forrest.)
One of my little grandsons called his other grandma Grandma and he called me "Forrest's mom." Or "the grandma who lives in Forrest's house."