This is not quite correct. If I have deduced this correctly, it works like this:
There are 2 matriarchs per zone.
In the save files, he has divided the wives and daughters by "zone", but obviously this would result in overlap between the two wives in each zone. So there are invented zones instead. The invented zones don't exist in-game, they just refer to one of the other matriarchs.
Islands is just the daughters of Morrigan the redhead. You find her in the Mountains.
Rivers is for daughters of Nephthys and/or Nut. They can be found in Deserts.