Actually, in ancient times...girls wed and had children young, because life expectancy was so low. This is not just in Biblical times, but for long after. And not just in Christianity, but across all religions. As people started living longer, children began having the luxury (for lack of better words) to actually have a childhood and be views as vulnerable and not equipped for such a task. And to add...there is actually no direct mention of pedophilia in the Bible. The Bible does go into great lengths against harming children, and perversion in general.
With all that said, Christians are not under the law of the old testament. So, things happened in society that was "accepted" by society that were not in the new testament. And we have to understand, just because something happened in society, does not mean it was commanded by God. Incest certainly did happen in the Bible. (Old Testament really.) In the Old Testament incest was allowed in society, because it was not genetically dangerous at the time or illegal. But God demanded new laws against incest under Mosaic Law. We were also less sophisticated in those times. There was really no education or theories about child psychology or sexuality whether deviant or not. Well, there was...but it was very simplistic and not rooted in understanding people at a psychological level. As time has progressed so has our ability to reason, develop insight into children and their psychology, and understand what is damaging to children and people in general.
Anyway, none of this has to do with anything. He molested his sisters and that's wrong. Period.