I Capture the Castle
by Dodie Smith

Against his brother's inclinations, he falls for and proposes to the elder of the novelist's daughters.

She is delighed - not because she loves Simon, but because she can now help her family with money.

Gradually, however, it dawns on the younger sister that she loves Simon, and she is incensed by her sister's casual attitude toward him.

But when the elder sister really falls in love, with Neil, then a double wedding is certainly in the cards, and so is salvation from the poor house.