[Sumeragi]

My theory on the Takanashi parents' disappearance:

Spoiler for Just my thoughts:

They did survive somehow, and decided to use the occasion to see how things might go while they were gone. It seems the Takanashi family has a large fortune, and the father most likely was worried what might happen should he be prematurely gone. Basically, the way the surviving Takanashi family members tried to get the kids for some reason seem to point to getting custody of the assets of Father Takanashi.