If you go to the Tantalus hideout at the beginning of disc 4, you will find Zenero and Benero wondering where Marcus and Cinna are. You've likely seen this before, but this is actually the beginning of a side quest. Now, head to Memoria and proceed until you watch a cut-scene or fight a boss. Then head back to the hideout to find Genero wondering where Benero and Zenero are. Then repear this process seven more times and every time you go back to the hideout, you will find a new member of the family. The last five are girls and the final one seems to be a child, as she is really small. Once you talk to her, Zidane will comment on the huge family and you will now find a new chest containing a Protect Ring inside the hideout. The next time you come back, the quest will simply start over, but there's not enough events to complete it twice.