Fudatsuki no Kyouko-chan is a pretty decent manga that just didn't live up to its full potential.
The pacing of the story is nice, but the level of development in the main premise is lacking. For a story that is based primarily on the interactions between sister and brother, the side romance element seems to be too overwhelming and forced. It feels like the author is desperately trying to "ship" the brother and Hibino out of fear of the taboo of the alternative (bro+sis ship), when in fact doing so would create a much more powerful and surprising narrative (if the sister was actually the brocon deep down). This is mostly because of Hibino's character development and interactions with the MC.
The artist very nicely draws a distinction between dere Kyoko vs. tsun Kyoko. Best example of this is the first time her ribbon came off. That super adorable smiling and apologetic face, when juxtaposed with her usual appearance, clearly conveys a sense of bipolarism in the character.
The MC's expressions are also well-drawn, so that the reader feels his anger, embarrassment, and even horror whenever the ribbon comes off.
The characters are decently likable, but their full potential weren't realized. The tension between Kyoko's two states and their relationship with the MC could have been explored in much more depth, especially with Hibino as fuel. However, because the author chose to develop a romance between Hibino and the MC, Kyoko's quirks became secondary. Added to that is Hibino's blandness as a character: She never really was much more than a passive person who wasn't afraid of the MC, and by some convenient reason he liked her. In a way, it felt like she didn't deserve him. On the other hand, because of her tsundere personality, it always felt as if there was something more to Kyoko's feelings towards her brother the MC, but this was glossed over in favor of a less "incest"-y manga.
Due to all of this, while the ending was fine, it felt a little bit like a cop-out. The central problem still isn't resolved. Kyoko dangerous nature is never fully addressed or tamed.
It was good. It gave me a few laughs, but nothing more. I did get hooked on it enough to read to the end, so it was interesting. Yet, due to the issues mentioned, it felt incomplete. It never reached its peak and definitely didn't end with a bang.
Could have been one hell of a manga, but ended up being just decent.