The First book is Mito's Sister, but to make it already confusing; there are 2 versions! One is the original one, and there is the rewritten one (not finished yet). I made a few mistakes with the timeline in the original one, and I knew I just could do better, so I decided to rewrite the story.

The plot is different, but they both connect to the sequels. 

The original one; 

The rewritten version; 

Go to the next chapter to see the sequels