As far as I know, Kai is the only member of the band who doesn't have siblings. Ruki has an older brother, Reita has an older sister, Aoi has both an older brother and an older sister, and Uruha has two older sisters. Hope that helps =) Edit: Actually, after Kai's 答案 in Shockwave, it would seem he does have siblings...he mentions a niece and nephew. Nowhere have I read about his siblings though.
Kai is the only child in his family, Reita, I heard has 2 sisters, Uruha has a younger sister, Ruki has an older brother and Aoi has one brother and one sister
