Usotsuki Na Cinderella summary:

When the wealthy Alfred Wingate dies, his personal assistant, Cynthia, is utterly confused. He has left her his mansion in his will! Cynthia thought she was in love with Alfred's grandson Graham. But when she finds out he is cheating, she breaks off the engagement. But the terms of the will won't allow her to refuse the inheritance, and she's stuck with the mansion. On top of all this, Graham's brother, Rick, treats her with contempt, believing she tricked his wealthy grandfather!