I've had this theory since chapter 5-ish, but I think there's finally enough evidence to put it out there. I think Ayako has special pheromones (kind of like that one yuri assassin manga that the name of slips my mind at the moment) that causes others to be attracted to her. I think her and the aunt were researching them in college to try to find a way to cure her, and somehow it ended with the brother dying. I think Ayako is humoring Asuka because she thinks that it's her fault and that she can't control her feelings for her because she's under the pheromones' influence. It's possible she's right (but I doubt it), and that Asuka has the same pheromones albeit in a smaller amount, for which Ayako is probably going to blame her attraction to her on. I figure end game is that they both get cured and find out that they do actually love each other after all; maybe the incestuous siblings will be the ones to point out that it's possible to just accidentally love each other?
