KBTBB: Their Kids/ Book 1: The Ichinomiyas

After being bought at the auction and falling in love with Eisuke, the MC (Miku) found out she was pregnant with Eisuke's child. That child turned out to be twins, a boy and a girl, and Miku decided that she needed to leave Eisuke for the twin's well beings. She left without a warning and raised the children on her own. 16 years later, one of the twins is bought at the auction because she broke an expensive item. The twins lives are turned around and just as everything seems to be calming down, a series of events gets everybody on edge.

A black market auction,
A dead body,
A secret to be exposed,
And everyone will go down for it.