Final scare/shot meaning? (SPOILERS)

The plot of the entire movie -- save one scene -- made perfect sense to me. I did have to watch the close-up shot of the body the police recovered from the lake several times just to make sure it wasn't his wife. One character even made a point of telling the others (us) to check out the expression on her face, but I had a feeling that meant check out which face that is. The fact that it was the first victim is also later confirmed in the dialogue.

When we see the final shot of John Lithgow dressed as Margot in the last scene, what is that supposed to represent? I thought it might mean that the daughter's personality had split, taking on one of the personalities of her father. Maybe it was the wife's personality split? I don't think he was really in the park because he just disappears into thin air. Beyond it being a great final scare I think I missed the point. Anyone know for sure?