You Lead, I Follow (2011)

Sinopsis

Niklas and Søren are old friends, and there is a clear division of power in the relationship: Niklas is the strong, Søren the docile. On a drunken night, Niklas decides to make a contest: Whoever takes a young girl with the "Anal Pro" sex toy has won. But as he makes Søren's 17-year-old sister Ida, part of the competition, Søren is driven beyond the edge of the sexual power play. Søren is horrified to see his sister brought into Niklas' sadistic power games, but is there anything he can do about it?