I didn't get what was going on until the last part of the movie. Of course it was edited for TV (which was fine with me), but I could not understand the girl coming up out of the water and the bugs all over her until much later.
By the time the youngest daughter was seeing 'her sister' with different eyes and was seeing and talking to the dead grandma, I got to thinking this could be taken different ways. You could imagine that grandma was also haunting them or that grandma was really this poor girl's conscience, telling her to get out and get a life while her mother slowly goes mad and is no longer in control of the household. I would have to see it again to get all the details but I don't know if I could take it once more!