There is a paradox there. Everyone would agree. Well, let's not exaggerate. Rather, let's say that much research indicates that children who are abused within the family are more scarred than those who are abused by strangers. That said, intrafamilial abusers are less likely to reoffend than extrafamilial abusers.
That is a paradox. If the sentence is based on the impact on the child, we should punish those who have committed incest more harshly; but if the sentence is based on the risk of reoffending, they should be less harshly punished. That is the paradox.