Chabrol's second film is a sort of mirror image of his first. Instead of a city dweller arriving in a small town where he spends some time with his old friend, a small town boy comes to the big city to study and spend some time with his cousin. The tragic redemption of that film's finale is replaced here with tragic irony, as (spoiler btw) the protagonist's single bullet intended to kill his cousin is turned on him.
I definitely liked Le Beau Serge more than Les Cousins, but there were some really interesting things about this film. It serves as a bridge between Chabrol's debut and his subsequent career, where he was focused primarily on thrillers, but it's also much more recognizable as a work of the French New Wave, both technically (where it is much flashier) and thematically, where the quirks of modern life are juxtaposed with the grandiosity of the cinema. Les Cousins is at its most contemporary in the film's pivotal party sequence, where the ominous gun is introduced and Charles falls for Florence. The characters in this sequence are delightfully modern, evoking Fellini's bombastic upper-class partygoers of La Dolce Vita a year before that film was made, and the nihilism of "kids today" shines through. The movie's almost love triangle, too, is reminiscent of a later film: in this case Jules et Jim, Truffaut's tragic love story. But Les Cousins doesn't reach nearly the heights of either of these films. It's a good story told by a smart filmmaker who had yet to find his voice.