Wanting onii-chan's D is a completely normal and expected state for a healthy imouto. That's just how the nature works and there is absolutely nothing whorish or improper to it. It wouldn't be an exaggeration to say that the right to being bred by onii-chan is a part of the natural law and a basic human right of every imouto.
The real whores are those filthy, uncivilized sluts that believe they are allowed to defile someone's onii-chan. All sexual acts that lack blood relation are inherently impure and sinful, nothing more than just an animalistic lust devoid of any meaning or substance. The bonds of blood are the strongest connection in the world, undeniable, unrelinquishable, present since the moment of birth. You went to the same class? He helped you once? Don't make me laugh, as if such a paltry forgery could hold a candle to the familial ties. Therefore there can be no love without the bonds of blood, as the true love, the perfect ideal, cannot be build upon an imperfect, inferior foundation.