[AP24]

I just found these links and if you can understand Japanese you will know who the sister really is.
Vol 8: http://d.hatena.ne.jp/kyoyatouko/20121005
Vol 9: http://d.hatena.ne.jp/kyoyatouko/20121224

Also it seems vol 9 is the last one. From what I can understand the little sister is

Spoiler for vol 8 & 9:

Konoe. In vol 8, it is revealed Miyabi is Shougo's cousin and Konoe is Shougo's sister. The family tree from the link to vol 8 spoilers shows the relationship between Shougo, Konoe, Miyabi, and their parents and grandparents. I think in the link it also says that Konoe is Shougo's lover so I guess Shougo finally had chosen Konoe (well it was obvious it would be her). They must have felt crushed about this revelation.

But there is good news revealed in vol 9. Konoe is Kumagorou's biological daughter but she's not really Shougo's sister because Shougo is not Kumagorou's biological son. For some reason Kumagorou and Shougo's mother Kanoko were unable to conceive a child together and Kanoko gave birth to Shougo through artificial insemination with another man. I expected that the girl Shougo would choose would be revealed as his sister. The author was clever in making Shougo not Kumagorou's biological son since the only way for him to be together with his sister is if they're not really related by blood. So in the end there is no sister because Shougo and Konoe are not siblings biologically and legally. Thus, they can get married and so it is a happy ending.

I might be wrong so I hope someone can clarify this.