"I never said you were mad, Bellatrix," she said. Narcissa was trying to see her sister's point of view, but they were both raised under the same beliefs how could it be that only she wants to keep those rules and appease her parents. "That's not the point. It's how it has always been done. It's part of our birthright and it goes against tradition."
When Bellatrix mentioned Lucius it was a low blow. She had already been afraid of falling in love with Lucius and knowing there was nothing she could do and she had to marry someone else or Lucius had to marry another. He was older than her and his family and himself could chose any woman in the world thanks to his money and status. If she were to be in love with Lucius she wouldn't know what she'd do.