The Dreamers (2003)

http://us.imdb.com/title/tt0309987/
A young American studying in Paris in 1968 strikes up a friendship with a French brother and sister. Set against the background of the '68 Paris student riots.

Brother casually help sister sexing with guy.

http://rapidshare.com/files/254796866/Dreamers_scenes.avi
67 MB