drown herself? And where was Nell during this? It looked so deliberate. I can't get past this part.
Also, the connection between Mary (Mrs. Sherriff) and Nell, seemed so instinctive that I wonder if they also are half sisters. If Nell was the product of a rape, in what seems like a really small town, it could be that Mary's father was the same as Nell's.
I don't think that part was meant to be taken literally. It was a visual representation of what Nell was experiencing: she realized she had to let her twin go. She had kept her twin alive within herself, but now she has to stand aside and let her sister walk into the water and disappear. She has to let that connection break in order to become an adult, and it's an incredibly poignant moment because that connection has been her constant her whole life. The movie never tells us how May actually died, though.
I think Nell's connection with Mary simply demonstrates Nell's ability to connect with others in an honest way that doesn't require language. As Jodie Foster commented, this movie makes the suggestion that language, which we value so highly, actually might make connection more difficult because it tricks us into thinking we know people by talking to them. With Nell, this kind of superficial connection is impossible, but this simply forces Jerry and Nell to go deeper to understand who she is. I would argue that there's also the issue of culture. Nell's grown up sheltered from our standards of "appropriate" behavior, which is why she looks straight at Mary and doesn't look away, which makes Mary a little uncomfortable at first, but this lack of self-consciousness on Nell's part lets her see Mary in a way those around her can't.
Also, I may be making this up, but I got the impression Nell's mother was from a different town and only moved to the woods around Robbinsville after the attack... I'll have to pay more attention to the newspaper clip next time.