Is it legal for an adult other then the parents to spank a child?

I live in the state of Indiana and have sole custody of my 3 children; oldest is 8 and the twins are 5. They see their father every other weekend. It has come to my attention that their father will allow his brother to threaten to spank our children when they do not behave. He actually will go across the street, where his brother lives, and have him come over and threaten to spank them if they do not behave. I have told the children that their uncle does not have the right to spank our children nor to threaten to do so. I have told their father that this is not acceptable and that he himself must get them to mind. He informs me it is none of my f-ing business how he chooses to displine. Am I correct in the assumption that no adult can legally spank our children other then the parent?