Blood Relation

There are N persons in a country. N is very big in millions.
Design a efficient data structure and algorithm to find out whether there is blood relations between given two person or say they have a common ancestor .


My solution:
Used a map //UID by Nandan Nilekani :)
struct person
{
struct person *father;
struct person *mother;
list child; //However may not be needed for this problem
string name ...i.e. //Add more
}

struct person * findAncestor ( struct person *a, struct person *b)
{
Take first node in stack.
While ( stack is not null )
{
Mark Node.
Push Its ancestor in stack;
}

Take second node in stack.
while ( stack is not null )
{
If marked return Node,
push ancestor in stack.
}
return NULL;
}

It has a drawback, i need to clear the flagged node before i apply the search algorithm again.
But till now the best i know.