Okay Willoughby was a cad and a bounder and took himself out of the running, but I do think that Brandon swooped in to take advantage of her rain induced fever which had left her a bit dazed and compliant.
It's a bit creepy that he falls in love with her because she reminds him of his long lost, 'fallen', dead love. Plus he's a bit intense, the laughs wouldn't be forthcoming and I'll warrant he expected her to do all kinds of dark shit in the bedroom.
Ideally she'd have had another couple of seasons in London and met lots of nice suitors or maybe even nipped across to Pride and Prejudice and married that nice Colonel Fitzwilliam.