I'm leaning towards the idea that someone might have taken Bianca, but Dad knows exactly who that person was and has his own selfish reasons for staying silent about that little detail.
Or Dad killed her. But the other option at least gives me hope that she may be alive, I don't want to contemplate in what condition, but alive.
What reason would dad have to keep secret about who that person was? Unless he is protecting his other childs mother?
jmo