After her date with her boyfriend, my sister comes home feeling downcast. Seeing her sister so dejected after failing at having sex makes me realize she's been masturbating every day. She even expands her pussy on her own! Unable to hold back any longer, I tell her that we should practice because "it'll make his big dick fit better." Soon enough, she starts enjoying the pleasure of incestuous sex more than just practicing... We start having harder, more intense intercourse without realizing it.