Family Tree

How, exactly, does the 'family tree' work? I looked in FAQ but found nothing.

when someone new joins by a hit from your bill and enters at least 5 bills they become your child… thus the family tree starts … hope this helps :)

And hopefully soon it will go the other direction as well. So you can tell who your hit-daddy or mommy is, or whether you have one at all!

Thanks shewolf and Pooga.