Answer:
It has long been established that fear gives people the courage to do things they ordinarily wouldn't do and Isabel in Chains is no different in that regard. For her, fear acts as a spur to do the right thing, whether it's defending her epileptic sister Ruth or rescuing Lady Seymour from a raging fire.
In a way, Isabel's numerous acts of bravery are a way for her to deal with the almost permanent fear that she must feel as a slave, constantly subject to the whims of others. In her situation, many would just keep their heads down and do what they could to make the best of a very bad job. But not Isabel. She understands that it's better to die on your feet than live on your knees. So she takes risks—extraordinary risks—that put her in harm's way, but which at the same time also allow her to recover some of the dignity and self-worth that has long been denied to her as a slave.