Squick: One of the higher-level Sharingan variants requires the user to do...something to his biological daughter. We only know that it's utterly depraved, given the source, and that Sasuke doesn't want it spoken of.
All of the Sharingan variants do this. Mangekyo requires to murder best friend. Temporal travel requires what's mentioned above. And as was mentioned during Snake Sennin's and Itachi's confrontation, other powers require to do something to parents, and possibly male offsprings.