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Chauvelin blackmails Lady Blakeney into helping him to lay a trap for the mysterious Scarlet Pimpernel by threatening to have her brother Armand arrested. Lady Blakeney of course doesn't know the real identity of her husband yet. I suppose one can't blame her. Life was cheap during the French Revolution and her brother would likely have been killed. Lady Blakeney might have felt that there was a chance that the trap would not work. Later , when she discovers her husband's identity, L. Blakeney regrets setting him up but at the time she did save her brother.