While watching this for the umpteenth time tonight, I picked up on something I hadn't noticed before.
In the scene where Manouche first visits Gu, while she's walking through the corridor of his building, turn the sound up: the climactic scene from Les enfants terribles is playing over the soundtrack. Nicole Stephane is giving her great "hypnotizing" speech with Bach's piano concerto playing in the background. It's as if, in some impossible way, the soundtrack from that earlier film were seeping through the door of another apartment as she passes. An appropriate reference, anyway, considering the sibling connection.
Isn't Melville wonderful?