This movie is NOT about incest!
I can't stand people who only discuss the incest aspect of Le Souffle au coeur. This isn't the main theme of the movie, and it's not the reason it should be watched. The incest is only a small part in the movie, and is more a metaphor than a real plot point: it was just a way to show his sexual awakening.
When I think about this movie, what comes to mind is Laurent sick and his mother dressed up in a costume, playing the guitar and singing to him. That was a beautiful scene, and really shows how much they love each other.
And there's so much more about it that could be discussed rather than that. It's about growing up, about the passion for arts and about how hard it is for a young kid become mature; the first loves, the lost of inocence (and not only in THAT scene) and (that's right!) of how a kid must get out of the shadow of his own Oedipus complex (in this movie, through a cartasis) and live his own sexuality.