Just like this series' other doujinshis before, this doujinshi describes the experiences of some characters which happens long before the current Gensokyo. In this doujinshi, the author seems to consider that the Aki sisters were no siblings at all, and at first did not even know each other... After they found their power could form up some balance, they decided to travel together and call each other sister since then.
Haruomi's stuff is always wonderful. But it's a shame that his work is rarely ever translated, and the rare times that it is, the translations are sub-par. I really hope this one gets a proper translation.