"She's your -- No, I didn't know." He honestly had no idea that Morgause had been her sister. It certainly explained why she'd been willing to stop everything for Morgana. Had she known they were family before now it might have even helped explain why she had helped putting everyone in danger, but she only just found out -- Merlin didn't want to think about it. He just shook his head.
The fact that she had found out from the show made him slightly curious about what else was shown that he didn't know about, but he still wasn't sure he wanted to try watching it again. It was just watching all the worst parts of his time in Camelot again.
"What had happened? Where had she been - Why didn't she try to meet you before? Did it explain?" Merlin did wonder why Morgause had waited so long if she had known Morgana was her sister. You had to wonder why she didn't bother before then. It wasn't until she was ready to try and attack Camelot that she -- Merlin supposed that was unfair. Maybe Morgause didn't find out until she'd come to Camelot either.