Here's a theory:
The parents divorced for the children's sake - not to split them up, but to get them together.
They didn't necessary want to split up themselves, but after seeing how close their children were, they decided to take action in order to make them "not siblings" and give them an opportunity to get even closer without having to worry about "incest".
They're keeping this plot a secret so as not to influence the outcome.
They're gonna leave their children alone to let the feelings stew and nature take it's course - whatever that may be.