It's almost like they were diverting us from the Arthur/Morgause line by revealing the Morgana/Morgause relation. I keep getting Aunt/Older cousin vibes from those two, and it's just wow. On that note, Bradley and Emilia are SO GOOD at the subtext! I mean really, they have sparks. Just in the same scene, you can SEE sparks flying, and that just builds up so much more story without the writers raising a finger.
That scene, with Igraine, Arthur, and Morgause in the same room was just wow. It looks like a family get-together! I think Merlin saw it. Really, I think Merlin's picked up on the not-very subtle relationship.
Now I feel I need to re-watch that episode now, and see things from that perspective :D