Board Thread:Chapter and Episode Discussions/@comment-4271790-20141108071032/@comment-25544939-20141111184435

Elveonora wrote: Actually it's possible. Maybe Sakura is infertile, so Karin had to provide the egg therefore is the biological mother. Sakura just served as a walking incubator for 9 months.

Or maybe Karin died during childbirth but I doubt it. So many questions still unanwsered in this series thought.