What is the name of this movie..?

It is one adult movie, and probably it's german movie. it's a story of 3 people, brother, sister and their friend. That friend started to stay at their home, and then he comes to know that they have sexual relation(bro & sis.). And slowly slowly he adjusts himself among them and they 3 creates group sex relation they used to bath together fully nude. I'm sure that the actress who has played the role of that girl is one of the celebrity mamber of 'list of the great germans'.