Oh, man, I started joking again. All joking aside, it is possibly the most relentlessly grim movie you will ever see. It is powerful, and depressing. The subject matter is stark enough to begin with, but it is filmed in the minimalist style of Tarkovsky and Bergman, with sparse dialogue, empty interiors and exteriors, limited background music, and slow pacing. The sun never shines.
Nobody ever smiles.
It's a horror movie where the horror is real. This storytelling economy also applies to the details of the characters. We don't know. Sis persuades the friend to take her brother's cherry, but at the last minute she interrupts them and prevents the act from being completed. Actually, I didn't understand the ending either, until i listened to the director's commentary.