I have been pondering this question for years and can't for the life of me figure out why. I am pleased that at least one other person had this question...I began to think I was the only one to even think of it. I've read Austen's reasons but they don't make any sense. He loved her and didn't want to hurt her!? He thinks allowing her to go ahead and marry Willoughby might NOT hurt her? Brandon admits knowing how wicked Willoughby is, I mean he's just seduced his teenage ward. And he confirms to Elinor that he DOES know he is extravagant and dissipated (intemperate in the pursuit of pleasure; dissolute, wasted or squandered, irreversibly lost) AND WORSE before Marianne was humiliated at the party. In fact it is likely he knew of Willoughby's true character BEFORE the Dashwood girls left for London. If Willoughby's rich relative had not had the presence of mind to confront Willoughby, he'd have very likely married Marianne...and Brandon would have stood by silently and watched. He could have protected Marianne from much humiiliation in front of her snobbish relatives and the world at large by saying something. He might have kept her from becoming deathly ill. He might have saved her from a marriage to an uncharactered man. I mean he already knows, firsthand, the outcome when a woman is married to a dissipated man...his brother. It's disaster and ruination. So WHY WHY WHY would he not tell Mrs Dashwood ASAP. How can we conscion that sort of behavior? Yet multitudes view Brandon as...the epitome of manliness, honesty and integrity, Austens best hero. Austen tells us he is always concerned with other people feelings. Personally I see it as a huge plothole in an otherwise good story. Austen tells us he's afraid he might not succeed...might not succeed at WHAT? IS this a valid reason to remain silent while a tragedy unfolds? He also tells Elinor that her sister might reclaim Willoughby...RECLAIM him? He already judged him dissipated, which by definition means irreversibly lost. SO how can he believe she would reclaim him. Indeed, if she had married him, she'd be "reclaiming" him every Monday morning after a weekend of debauchery. Besides, is this how women want to be thought of? As someone whose job it is to "reclaim" faithless and philandering men? Willoughby is selfish and cold-hearted, vain and extravagant...yet Brandon tells Elinor he thought her sister might reclaim him. I have yet to hear any reasons that justify such negligence and carelessness toward a trusting and honorable family as the Dashwoods.