It requires a lot of obscure stuff to have happened before you can get everything, and most people won't even check after the first couple.
From the video description:
"This is a side quest on disc 4 that was unknown to most FFIX players until yesterday when a topic popped up on the FFIX message board at GameFAQs. It was discovered by The_Kusabi_ (through the Ultimania guide) and confirmed by MysterPixel. Naturally, I had to check it out and I decided to record a video.
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. These are the events that move the quest forwards:
1. The beginning of Disc 4.
2. The battle with Nova Dragon.
3. The battle with Maliris.
4. The flashback of Alexander.
5. The scene where Zidane talks to himself.
6. The flashback of Dagger on the boat.
7. The scene with the giant eye in the sky.
8. The battle with Tiamat.
9. The scene with the two planets colliding.
10. The scene where Quina thinks he/she is drowning.
11. The scene where Garland talks about memories.
12. The battle with Kraken.
13. The scene about Gaia's birth.
14. The battle with Lich.
15. The scene where Garland talks about the beginning and the Crystal.
16. The scene where the party enters the Crystal World.
17. The battle with Deathguise.
It's no surprise people have missed out on this, considering how obscure it is. It also makes an Excalibur II Perfect Game more complicated. New things are discovered in old games all the time, but it's usually glitches or obscure tricks. This time, we got a whole new side quest, a treasure chest and seven characters! Needless to say, my mind is blown."
Found on NeoGAF: http://www.neogaf.com/forum/showthread.php?t=568409