Shion looks older although she is Mion's younger sister. Maybe Shion and Mion are the heroines in Watanagashi Hen while Rena is the heroine in Onikakushi Hen.
The game seems to be a normal visual novel about dating with girls up to now (Subchapter 5). But I clearly know that something horrible will happen definitely if I continue playing the game.
We are a cute and loving international community Ｏ(≧▽≦)Ｏ !