Narcissa had turned before Bellatrix had finished. It sounded as though Bellatrix meant what she said, as if she believed it - as if the Dark Lord's death was not the end of her world, but a disappointing event that let her return her attention to her family. And Narcissa wanted to trust what she was feeling.
"That's the opposite of what you used to say," she said, taking half a step towards Bellatrix again. "So do you see why this is so difficult? I ... I do know it's not you, but it was someone very much like you ..."
No, Bella wouldn't understand, and making her understand would require handing over too many details. Narcissa would be perfectly satisfied for Bellatrix to never hear about the Malfoys' decline in fortunes.
And yes, Narcissa had always been aware of what Bellatrix could do. She had known that her sister was violent and cruel, and that she could delight in depravity in a way Lucius did not. But until that fateful time when Lucius had returned from Azkaban and Draco from Hogwarts, Narcissa had always been quite sure that she was secure in her safety, and would never find that malice directed towards her. Bellatrix had always been a protector, not a foe, despite their adverserial irritations.