^^Except that — in the actual Oomuro-ke manga — Sakurako doesn't seem to notice her sisters thinking she's an idiot nine times out of ten.
I'd actually lay odds that there were two things that soured their relationship — Himawari's boobs. No, really. Himawari developed early, making Sakurako jealous — and then complained about it, which exacerbated the situation. It's the single topic that sets Sakurako off the most in the series — not Himawari's size, but Sakurako's lack of development comparing herself to her; every time Himawari points out the drawbacks it feels like ungratefulness and mockery to her, especially when she phrases it like, "You're lucky you don't have to deal with this".
That said, Sakurako and Himawari still do everything together and make each other the centers of their lives. They just squabble.