Generally speaking, the eldest child is named for grandparents and often they appear as sponsors. Check out the actual baptismal records, since it often indicates the actual relationship of the child to the parents.
Older siblings were also often god-parents to the younger siblgins, etc. Younger aunts and uncles, etc.
For younger children in large families, it can be whoever was still available. But godparents is a very special relationship recognized by the church. So the names of the godparents always indicate a close relationship between the parents and godparents.