I am rather satisfied with the ending. While the drama wasn't as explicit as some may have hoped, I found this manga close to suffocating at times, because of the odd dependence that existed between the siblings. I think their relationship was anything but healthy, although not in that way.
Kyoko needed Kenji to survive, he was the only one who knew about her (apparently dangerous) condition and thus the only one who could help her with it. Unsurprisingly, this seems to have frustrated her greatly, but at the same time the prospect of her brother becoming busy in other areas of life (like romance) bothered her just as well, since, once again, her very survival depended (and still depends) on him. On Kenji's part, as others wrote below, the story seemed to go out of its way to make his life a living hell, but a sizable portion of that hell was actually inflicted upon Kenji by himself, due to him worrying about his sister. Consider this: Kenji willingly made himself a social outcast so no one would think ill of his sister when he was constantly fussing around her or when he tried to keep potential boyfriends away (for their own safety!). Hibino was the only one who could see past his delinquent persona.
What has changed in the ending is that their relationship moved in a somewhat healthier direction. Kyoko still needs Kenji, and Kenji still worries about her sister, but both have understood that this does not mean that Kenji has to ruin his life in the process, and even if a significant other enters the picture, his bond with his sister will endure. It's rather obvious that the manga was axed, so not all plot threads received a satisfactory ending (the student council president's infatuation with Kyoko was simply abandoned, sadly), but I feel the main conflict received proper closure, and I'm rather happy about that.