For major horror/movie buffs- What's the title! ?

About 8-9 years ago I've seen this pretty good psychological horror film with my cousin, but we don't remember the title and I really want to re-watch it, as it was pretty good (at least that's how I remember it. I think the main character, a woman psychologist or smth like that is going to her cabin (it's winter time) with her husband and younger sister. I should say there was no famous actors in the movie, at least I think it was lesser known actors. When she gets to the cabin, she catches husband and sister having sex. This is where the plot thickens and I have holes in my memory, but I remember there are burglairs, and they come on snow scooters and terrorise the trio, making the husband and sister have sex again in front of the main character. I think everyone dies except the main character, and it later turns out that there were no burglair-criminals, but when she saw them having sex she had a freak-out and imagined the whole thing and killed them, there's a guy talking in her head, she has visual hallucinations. Anyway, at the end she cuts off her finger with some sort of tool cause the guy she hallucinates tells her to and that's how it ends. Thanks in advance! ;)