>>717095198Real answer: It actually goes way back, Kirino initially admired Kyosuke BECAUSE he wasn't a simpleton, he was an overachieving kid that always told her to rely on him, and this made her develop a brother complex very early on. Eventually, this is what inspired her to also become an over performer, and that's why the Kirino of today is such a high tier woman.
Then the whole Manami thing happened and that bitch basically convinced Kyosuke to give up his ambitions and "be normal" like her, while at the same time shaming Kirino by calling her a weirdo and saying that her attachment to Kyosuke wasn't normal, this caused her to repress her feelings, and THIS, is what lead to her feelings eventually becoming sexual in nature, replacing the lost bond with her brother with eroges to give her the illusion of being in control.
Naturally, when the actual series began their bond was already strained, but it was clear that Kirino still desired that connection baxk, it's just that she couldn't be normal about it anymore, and anytime Kyosuke did something for her in the series, it slowly reminded her about the boy she once admired, leading to her feelings coming back, with this additional spark in them
The light novel actually covers a lot of points that the anime didn't, if you read it, it WILL make sense why Kirino is in love with Kyosuke.