I dispute the Westermarck effect.
But anyway, I see the story as the Kousaka siblings getting over the issues they had between them which includes Kirino's "mutated" feelings due to Manami's interference. As I mentioned earlier, it was likely that Kirino's feelings for Kyousuke was more akin to puppy love that a girl or boy might have for their older sibling of parent, but Manami's way of getting in the way most likely stalled the natural development and led to the current situation.
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