The Wild Fawn by Mary Imlay Taylor

THE CARTERS, after a few days, tried to settle down and get used to it, but the new Mrs. Carter never quite let them do it. She kept taking them by surprise, with a kind of flying grace that left them speechless. She was making rapid conquests, too. Leigh had become her slave. He followed her about in an embarrassed state of subjugation, and spent long hours in his room writing sonnets, dedicated to Fanchon, or, more often, to "another's wife."

Emily, meanwhile, was making some curiously subtle alterations in her own appearance, as yet undiscovered by her anxious mother. She had successfully negotiated a loan of three dollars from Daniel -- for purposes unknown.