Kiyoshi believes that Obanban is a substitute for his mother because the bachi from Obanban has fallen on his mom. Obanban didn't practice traditional ways when it cam to relationships. She didn't wait the mourning period of forty five days she eloped right away and then again after that in an untraditional manner left him and married another.
This is why she was looked at as the black sheep. Whether someone likes it or not there is a family member that is always looked at as "black sheep" of the family. With so many old traditions, rules, and restrictions it was easy to get this title back then especially when dealing with such a old established culture. Obanban also seemed like a mother figure the way she interacts with mama and the kids. Her maternal instincts seemed like they kicked in while mama is sick.
