mirror object based on parent transform

I have two spheres, children of an empty parent object. I am moving one object and want the other to follow the movement in reverse.

This is working based on world coordinate - how to I make it work based on the parent objects coordinates?

function moveOtherHandle()
{
	newPosition = transform.position;
	otherHandle.position = -newPosition;

}

Assuming the code above does what you want in global space you can use Transform.localPosition:

function moveOtherHandle()
{
    newPosition = transform.localPosition;
    otherHandle.localPosition = -newPosition;
 
}