I didn't read Kanata's route a second time, but after watching Ep17, a certain scene in Haruka's route actually is even more meaningful in that regard:
Spoiler for Haruka's route:
Haruka's revelations in the anime made me think something was missing and I remembered how "crazed" Haruka looked in the game (however bad Hinoue can be with proportions, expressions are usually accurate with her).
From there, I realized that Kanata's comment that Haruka would break at some point is indeed true: without a catalyst, Haruka would be unable to find a scapegoat for her resentment and frustration, to the point she would definitely conclude she is indeed a child that no one would have wished, precisely because "no one is at fault, therefore, her own birth is the issue".
As such, the moment Haruka pressed her hatred towards Kanata (instead towards the Saigusa and Futaki), the latter was basically stuck between a rock and a hard place: 1) she has to maintain the current situation, pressed by her own family that doesn't want to have any murderer offspring 2) being Haruka's ally will not only be spelled as pain for her, but also render Haruka's position as "no longer the victim".
With such equation, there is no way for Kanata to help directly Haruka from her own position because Kanata is exactly on the "good side of the fence" even if Haruka doesn't know the actual harshness for being the "chosen" twin.
As result, Kanata has to act as the bad guy, in order to "please" both her family and Haruka at the same time. Without such pretense, Haruka would lose the only way to vent her anger, which would make her crumble sooner or later. Assuming Kanata tries reasoning Haruka there, it would not work because 1) Kanata has "no reason" to lecture Haruka, because the former was chosen, therefore she is de facto "not understanding Haruka's pain" 2) even if Haruka believes her, it would just be even more tragic for the latter.
This explains why Kanata had resolved herself to play the bully, which also explains her "cryptic" quotes about happiness/unhappiness balance, and how people feel they are the "most unhappy" compared to others.
With such reasoning, it becomes apparent that Kanata has basically "no choice" to speak of. And because of the circumstances, she can't let Haruka know her own true feelings regarding the matter.
This surely explains why Kanata's route plays way differently, as it branches you with a "what if the twins were still caring for each other" scenario.