Akuyaku Reijou wa Single Mother ni Narimashita Futago wo Hikitorimashitaga Koushaku-sama Kara no Dekiai wa Souteigai desu

Estelle, once a noble duchess, is cast down to commoner status after the crown prince abruptly ends their engagement—unaware that she's been reincarnated as the villainess in a game. Determined to live quietly, she moves to a neighbouring country and begins working at a small restaurant. Six years later, Estelle is running the restaurant while raising Coco and Mia, the twin daughters of the late owner. Her peaceful life is upended when Duke Frederick, who claims to be the twins’ older brother, suddenly appears. To protect the girls from being caught in a bitter inheritance dispute, Estelle agrees to pose as his lover.
Source: MU