Responses
I don't think that every story should have a complete explanation because of two things:
1. There may not be a known beginning for her and her sisters. They existed so long that it could be forgettable. Actually, they are the first so that means they were born/made that way.
2. Their lives stretch to eternity so there's no actual ending. The other two sisters didn't show and probably long forgot the game or is fighting it in a completely different country.
I think the ending feels incomplete on purpose. We find out that the whole story between the 3 only happens because of Miwako and her sisters’ game. I think the point is that we only see their story and how it ends plus the main character’s resolution. But I wished there was maybe one or two more chapters to show the main character’s thought process about the whole situation (Miwako, the blood race, Chika and the other guy). I also wished that there was more backstory regarding the sisters but maybe we aren’t meant to understand them (or the mangaka could be planning something else?).