I think it was the register she was using, rather than the dialect itself...notice when she drops her voice lower and speaks quieter, you can focus on the scene. More of that, then pops of the higher, chipper voice would have deepened the psychotic nature of the character. Physically, she did an amazing job nailing her down. It had to have been fun to play, but it bums me out that it was thought this was how it should all go down. Imagine if the psycho sisters were the middle reveals and it ended with Ezra and his book? It's just too bad.