The retarded writing continues. This is the problem with Sara's perspective:
"I want to keep smiling with everyone forever."
So she says...even with Kurumi in mind. Said vertical surface came out of nowhere, as far as Sara's concerned: Sara doesn't know her, and Flatchest only recently showed up to do one thing--turn Sara into a cucquean before her relationship could even start. Worse, Sara is characterized as having such good insight as to be able to detect a lie just by being told one--yet, for some reason, she can't detect the malevolence borne by the flatchest for Sara. Sara's such a good girl, she wonders if wanting to keep her beloved to herself is selfish...all while Flatchest is waiting for a chance to remove Sara from the picture.
Again, I have no doubts whatsoever that Flatchest exists because Mori was too weak to oppose his editor. If MC-kun takes both Sara and Kou, it'd not only make sense--because it has precedent in the source doujin--it'd be ideal, given the relationship all three have, particularly that between the sisters themselves. It's Flatchest who stands out like a fake thumb, and she's forced in because of the foolish desire to pander to androgyny fetishists, in whatever form they come. The result is the nonsense in this arc.
The nonsense or, as the narration puts it, the (curveless) "shadow" cast over Sara's, Kou's, and MC-kun's future.