I think it's common to take on the accent of a new environment. Ten years in San Diego drastically morphed my Rhode Island accent into something new and different. Being back in RI for fifteen years has morphed it again into something even stranger.
I don't know of any recordings of Edie as a young girl but she grew up on Park Ave, not Long Island. I'd guess that she had a much more refined finishing school accent, probably closer to Jackie and Lee's, that became mixed with that distinct Long Island accent. I'm just supposing. Bit I can't imagine her speaking that way at Miss Porter's school and getting away with it.
There is one thing I find very interesting about the way Edie and her mother speak. Everything is described in absolute terms. Everything is "completely wonderful" or "completely awful" or "absolutely brilliant" or "entirely wrong." It's all extreme black or extreme white, nothing is just okay or a little annoying. It's either a disaster or it's the best.
I wonder if that's a class thing,where everything had to be perfect or it was unacceptable, which is ironic considering the conditions they lived in. Or is it just the particular penchant these two had for high drama.
Art has a double face, of expression and illusion.