You're SUPPOSED to get them in gongaga, but the character who gives them to you was deleted. the "Wife and daughter" both live in kalm, but having the letters doesn't change any dialouge. However, the lines form the dude in gongaga are still present in the games files.
The sub-quest was abandoned either due to time constraints, or they just didn't feel like it.
People think this is part of an abandoned "Aeris resuretion sidequest", but i dont see how a random dude from gongaga could help bring her back.