[dargueta]

Since both children have the same parent (eventually), just compare the parents recursively, like so:

struct Node
{
    Node  *parent;
    int  value;
};
 
Node *getParent(Node *childLeft, Node *childRight)
{
    Node *currLeft = childLeft;
    Node *currRight = childRight;
 
    while(true)
    {
        //check for nulls
        if((currLeft == NULL)||(currRight = NULL))
            return NULL;
        //check to see if the parents are the same
        //if so, return the parent.
        if(currLeft->parent == currRight->parent)
            return currLeft->parent;
        //parents aren't the same, continue up the tree
        currLeft = currLeft->parent;
        currRight = currRight->parent;
    }
}