Since Yuuka's mother died at an early age, Yuuka turned out to be the one taking care of her child-like father. Right before high school started, Yuuka's dad announced his remarriage with Satou Masami. Masami-san turned out to be a reliable, responsible person, so Yuuka finally thought she's off the hook. That is, until she finds out about her new older brother... (Source: The Little Corner)