I'm not so sure that Riza would be able to guess so easily who is under the mask.

There are probably a few other other brothers and sisters in the winds. Essentially any future PC participants in this game will have such a character, so there are likely 5 to 10 more brothers and sisters out there who haven't shown up in Amber for quite some time and are considered 'lost' until they show up. Just because we don't know their names yet, doesn't mean they aren't out there. Both Riza and Vincent came about this way, so it will likely happen in the future again.