This manga starts off pretty slow, with most of the humor being focused around the protagonist being incompetent and hated. It then transitions to actually caring about the main mystery, which pushes the plot forward causes the story to pick up tremendously. The main character begins to act in a way that is more appealing, and the humor also improves. However, they decide to attempt to sweep the solving of the mystery under the rug, and go back to the previous status quo of not knowing who the sister is in order to keep the story going rather than end it. This reversal felt pretty forced and awkward to be honest. Still, once it gets over the strange feeling hump of doing so, the manga is pretty good, in that the story continues to move at a good pace, the main character is still appealing, and the humor is also solid. All of this leads to a conclusion that is pretty much exactly what you would expect it to be, but it's pretty sweet at being so and I was satisfied with the ending. The art is pretty good. 

tl;dr: A weak start that turns into a pretty solid rom-com.