Thank you, toonia. I highly doubt the 15yo got up in the morning and said, "Gosh, I think I'll molest my sister today". Behavior like that usually has warning signs, and after a cause. Molestation is never justified. Neither is murder. Murdering your son doesn't happen overnight, either. More than likely, Dad's affection and relationship with Jr died a long time ago.
It's just a sad, farked up situation all around.