The Ending *SPOILERS ABOUT THE ENDING*

I always interpreted the ending as they had all done things with Lucy, and that, in the end, sending her away would run the risk of her exposing all of them. So, rather than losing their upscale way of life, they simply try to move on and allow Lucy to stay. The ending is very vauge, but that's what I got from it.

I just watched it and I came away thinking that in the final scene, when Lucy is dressed up to go out with them, the Doctor and the wife were at that point "stuck" with her, to avoid exposure of the wife's inappropriate relationship with Lucy. I didn't think the father or the son had been with Lucy. It seemed to me that the film gradually presented that the wife was not doing Lucy any good, although I think it was obscured till close to the end. There was a pretty good hint at that, in the scene that showed the wife fondling Lucy and the reaction in Lucy's face. Then near the end, shown more clearly, by the wife's refusal (and outright disgust) when the Doctor tried to have sex with her. (I didn't read the book.)