Question

I assume the other kids weren't abused. Duncan was the only non-biological son therefore maybe that's why his stepfather focused on abusing him. Perhaps he refrained to commit those vile acts to his own children. Perhaps...

But who knows? Duncan Roy himself said that he ommitted on purpose several crude and gruesome episodes with his stepfather, hence maybe he ommitted also his stepfather's sexual abuse against his own kids. But I doubt it. Me thinks Duncan would have tell it or at least implied it.