Sister actresses seems rare, even for twins. I'd settle for that, but I still think having A&M played by the same actress would be the best bet, considering how different they appear. Then you could cast a sister for Sarissa down the line.
A&M would work better in a purely storytelling way, since she is wearing a different face at first and only slips back to the near-identical face once Harry guess her real ID. But then you've painted yourself into a corner with Sarissa, because Sarissa needs to look /more/ like Maeve than the Cousin Aurora does, but not so much that you'd guess at the opening of CD.
If it were me, I'd file this under elements that just dont translate well from Book to Screen, along with MC's that have close to 18" of height difference. Since the physical similarities between Aurora and Maeve are not particularly plot relevant but would require casting two Aurora's at minimum, and limit the SK reveal bit with aurora to her shifting her eyes to the same Green Cat-eyes to match Maeve (which was the actual detail Harry seized on). IMO that would preserve the important elements while simplifying the casting challenge, especially when you consider that Aurora might potentially be cast a full decade before Sarissa, who ultimately is going to be the more critical casting challenge.
Also, another reason why (for the screen) Id want different actresses for the Ladies is to better contrast the similarities of Mab and Titania who I DO want to be the same actress. They are supposed to look so similar they are like Clones rather than just twins:
Oh, and if I hadn’t known better, I would have sworn to you that it was Mab standing there. Seriously. They didn’t look like sisters. They looked like clones.
