Answer to why the younger brother broke up with his boyfriend
Olivier broke up with his boyfriend in the third part of the movie for no reason at all. Read on.
I bought "Le clan" [Three Dancing Slaves] from fnac.com, a store in France. It came in a boxed set with the dvd "Les premières neiges," another film by Gaël Morel with his friends Stéphane Rideau and Elodie Bouchez. On the US released dvd of "Le clan," the only Bonus Feature is a photo gallery. On the French release Bonus Features there's the theatrical trailer, a sneak peak at Morel's next movie, the music video, and a 45-minute "Making of" featurette with interviews with the director and stars.
In the interviews, Morel (the director) plainly states that there is no reason whatsoever for the break-up between Olivier, the younger brother, and his Arab boyfriend, Hicham, other than Morel wanted to make the third part of the movie melodramatic.
Morel stated he has 3 favorite movie genres and he incorporated them all in this movie: the first part was American-teenager style, as he called it, with the guys all doing adolescent guy stuff; the second was slice-of-life, centering on Christophe's return home and getting a job and the conflict with his brother; and the third part he wanted to make a melodrama. Morel is very keen on melodramas, and commented that when he makes a movie he always keeps such movies as "Splendour in the Grass" and "A Summer Place" (two heavy-duty melodramas) in the back of his mind, along with everything he learned from André Téchiné.
In order to make the third part of the movie, Olivier's story, melodramatic, he had to make Olivier break up with his boyfriend, so he just arbitrarily wrote it that way, no explanation needed. Morel just made it happen to suit the film. He wanted the viewers to feel sad, hence the break-up, and then all the audience needed to feel happy at the end was the younger brother finding another boyfriend, which he also did. And the Arab guy was going to be happy too, having gone to Paris, living his life away from a group of people he didn't actually belong to, even though all through the movie he thought himself as one of the brothers. Which he was not. It took Olivier breaking up with him to realize that, and that his life was elsewhere.
So that's the only reason for Olivier breaking up with his boyfriend: the director wanted to make things more melodramatic.
A little bit of trivia about the movie from the interviews: although it was set in the Summertime during a heat wave, it was filmed in late October, and every time the actors were in the water, either at the lake or in the canal, they were freezing cold, and the reason they moved around and jumped around so much in any water scenes was to keep the camera from seeing how much they were shivering. In his interview, Kechiouche said the actors had "ice on their teeth" when filming the canal scene.