I wanted to like this, but from the time the guest turned up I just couldn't understand why they would tolerate and accommodate her. I suppose the terrible attitude was needed to make the action at the end in-character, but prior to that I just couldn't understand them keeping around the absolute worst idea of a teenager - bratty, selfish, ungrateful, entitled and antagonistic. It would be hard enough to put up with if you were their parent, but as the couple had no link to the irritant they had no obligation to tolerate her attitude and should have just turned her away.
And how did she, whilst also pregnant, get out there in the isolated location in the first place?

Regardless, with that stinking attitude she'd be left where she was by any reasonably thinking person I could imagine, who is trying to live in isolation.
So this character spoiled the film, for me.