Similar to a '70s movie *SPOILERS ABOUND*
Sorry, saw the twins thing before in Robert Mulligan's 1972 adaption of Tom Tryon's novel, "The Other." Didn't think "Goodnight Mommy" was direct plagiarism but it's definitely informed by Tryon's story.
One of the early twin reveals are identical and there's even a cat.
"The Other" was better, by the way.