Otherwise have succumbed and am reading my first Amelia Peabody. Also Komahoshi, which at the end of vol 10 suddenly reverted to the desperate edgy happenings of the first volumes after about five tanks of Happy Skool Daze rabu-rabu.
The deal with the Kashiwagis and the Hosakas is this.
The oldest daughter of the Kashiwagi family and her husband, evidently an adopted son-in-law, had two children, a boy Masaya-I-*think*, and a girl Ayako. Mr Kashiwagi died. Mrs Kashiwagi remarried, to the second son of the wealthy business Hosaka family. The elder Mr Hosaka disowned his son, for whatever reason-- thwarting his wishes, I assume. The couple had a son, our problem child Kiyomine. So Ayako is a half-sister, not a step; but I still don't get why she seems to appear first of all when Kiyomine is a kid, nor why he calls her by name; and the fact that she's a half-sister makes his akogare just a little yuck to my tastes. Unless of course I'm misreading that bit as well.