Synthesized answer
The key concepts revolve around a passionate but penniless love affair between Richard Curwen and Antonia. Both Richard and Antonia desired a luxurious lifestyle but lacked the financial means to achieve it [1]. This led Richard to marry Eloise, Antonia's wealthy cousin, due to her striking resemblance to Antonia, a similarity Richard intended to exploit [1].
The passages do not provide enough information to fully explain the key concepts beyond this setup. It is unclear what Richard was counting on by marrying Eloise, nor is there information about the consequences of this deception or the ultimate fate of the characters.
Synthesized from the book passages below. Chat with the book on Feynman for follow-up.
From the book
Title: Bride of Moat House by Norah Lofts Description: It began when he met Antonia. Richard Curwen fell in love—wildly, passionately—and the lovely Antonia returned his love. But Richard and Antonia were penniless. And too spoiled and selfish to live as ordinary mortals. They wanted the best. The very best. And so Richard married Eloise, Antonia’s frail but wealthy cousin. The resemblance between Eloise and Antonia was astonishing. In fact, most people couldn’t tell them apart. Which was exactly what Richard was counting on.