I wonder if sometimes Boyfriend-kun lets just the two of them make out while he sits in a comfy chair and watches. You know, to get the yuri experience...
Well hey, at least even by that end, the two sisters still get along well...I mean, they even share the same boyfriend. Also, this guy certainly lucked out. He basically got two girlfriends out of this situation thanks to his first girlfriend basically convincing her sister to join in... Not that much 'convincing' was really needed.