I took a look at the web comic and this gets pretty dark. I hope the serialization doesn't follow the web comic that closely. I'd prefer a little less suffering.
This is a translation of an excerpt from Kamejiro's fanbox:
When I was first offered to serialize this manga, I thought I'd set it chronologically after the Princess Side. The incident at the capital could be revealed through flashbacks while the pair went on a heartwarming love comedy adventure for two. I happily drew up drafts of the first 3 chapters of this, and during the serialization meeting this idea was utterly shot down.
My editor suggested restarting from the very beginning to allow new readers to better enjoy it. Which they were completely right about. So it was settled I would be drawing up a reconstructed story taking into account everything from the Princess Side on twitter and pixiv, the Sister Side on fanbox, and the doujinshi prequel.
I plan on following the chronology of the self-published version, but things might progress a little differently with new events mixed in. Those of you who already read the self-published version will have fun with it too... I hope. Since I have the opportunity to start it all over, I'd like to make it even more exciting than the last time. I'll try my best.
So far these 2 chapters have featured a lot of new material. A few notable plot changes include Sara not being able to summon Sylpheed and Sylpheed's memories only being visible to the host. Also Natalia used to be much less paranoid of Sara.
But I'm confident the story will play out the same. I'm not so sure to trust the "heartwarming love comedy adventure for two" part, but things can only go up from where it left off.