Since I doubt that will happen: Does she stand her ground and tell him what she wants or is she scared off by the one-eyed monster, resting the fate of their relationship on MCkun seeking her out to finish the conversation?
"I can-and-do fuck my demon girl whenever I want and sticking my dick in the landlady would probably endanger my very comfortable living situation; however..."
-MC, probably
Sister is willing to be in a polycule, wife is not, husband doesn't know what his sister is talking about because he cannot find that word in the bible.
Or just doesn't recognize the sister from the photo because it's been a while since she saw her.