Morgana only knew who Nimueh was through watching the show. Nevertheless, if she had been the High Priestess, then she was the one who had raised her sister. "That's what Gaius said." She answered. "He never mentioned her by name. However, who else could it have been?" The question was rhetorical. Morgana didn't know much about the Old Religion. Nothing except the very little she had happened to read of it when she was just a child. And that was difficult to remember. It'd been before she turned ten, her father hadn't share Uther's hatred. And finding books about it in Camelot, let alone in the castle, was completely out of the question.
When Merlin continued, Morgana watched him carefully. Still slightly, knowing what he wanted to say but grateful that he didn't. Uther had made Gaius swear not to tell her who her sister was. Because he hadn't wanted her loyalties torn. Unbeknowst to her, she had chosen. Morgana knew she would have never meant to put a line between herself and Arthur; however, if she decided to act against Uther she might as well have declared action against Arthur because there was no way he would turn his back on his father. Nonetheless, here, it wasn't an issue.
In her mind, in the time she was from it had never been an issue in the first place. Because besides the fact that Arthur would side with his father, she would never have any problems with him beyond that. Morgana respected Arthur far too much to actively harm him ... outside of the Uther situation. It was Morgana's belief that with Uther out of the way, that neither women would have had issue with Arthur or Camelot. But she didn't say that. "I have no quarrel with Arthur." She said, after a moment. "I never have," Morgana sensed that Merlin would know that she meant in the way she had one with Uther rather in the childish ways that she and Arthur still managed to fall into, even though they hadn't been children for years. "and I doubt that I ever will. I can't ... " She couldn't promise him what she would do if they ended back in their time. With what she knew now. Could she choose between her blood relative and the boy who'd been like family to her for most of her life? "I won't hurt Arthur." That was all that she could give him for the moment, and Morgana hated that. She owed both Merlin and Arthur a lot. The situation in the dungeon wasn't the first time Arthur had gotten her out of a situation she'd dug herself into. Morgana knew that she owed them both more than that and if the situation had held any other people besides Morgause and Uther; she would have sworn her own solemn oath without even blinking an eyelash.