Hitherto, Rowena had sustained her part in this trying scene with undismayed courage, but it was because she had not considered the danger as serious and imminent. Her disposition was naturally that which physiognomists consider as proper to fair complexions, mild, timid, and gentle; but it had been tempered, and, as it were, hardened, by the circumstances of her education. Accustomed to see the will of all, even of Cedric himself, (sufficiently arbitrary with others,) give way before her wishes, she had acquired that sort of courage and self-confidence which arises from the habitual and constant deference of the circle in which we move. She could scarce conceive the possibility of her will being opposed, far less that of its being treated with total disregard.
Her haughtiness and habit of domination was, therefore, a fictitious character, induced over that which was natural to her, and it deserted her when her eyes were opened to the extent of her own danger, as well as that of her lover and her guardian; and when she found her will, the slightest expression of which was wont to command respect and attention, now placed in opposition to that of a man of a strong, fierce, and determined mind, who possessed the advantage over her, and was resolved to use it, she quailed before him. (23.28-29)
When Rowena faces down De Bracy, she first appears strong, tough, and unafraid. However, Scott tells us that the reason she seems so tough is because she's not used to anyone not immediately obeying her wishes. When De Bracy keeps pushing Rowena, she breaks down almost at once. She has almost no resistance to bad treatment, because she's not used to it.
Compare that attitude to that of Rebecca, who is accustomed to prejudice and harsh words. Because Rebecca doesn't expect people to obey her or treat her well, she's much more resilient when she has to deal with cruelty. In a sense, Rowena's sheltered upbringing has really damaged her. Rowena appears relatively weak during this whole siege at Torquilstone. By contrast, Rebecca is strong enough to face even being burned alive without compromising herself or giving in to her tormentors.