...or nearly engaged.

This is something I have actually never noticed until this read. In Ch. 16, Marianne is observing Edward's peculiar behaviour when he comes to visit them:

...it ended, as every feeling must end with her, by carrying back her thoughts to Willoughby, whose manners formed a contrast sufficiently striking to those of his brother elect.

This is from Marianne's perspective, and the choice of words 'brother elect' shows that she is imagining that Edward and Elinor will be married as she and Willoughby will be too.