"Sure."

The subtle relaxation of his face might be at least half deliberate, but that doesn't mean it's faked.

(With everyone else, Trowa can either rely on Quatre to tell when it seems appropriate, or allow them to deduce it in their own time. He's aware that telling his sister is not really a conversation he can get out of. As such, better to do it while she still considers it a more or less timely mention.)