Clement Virgo’s “Brother” is the kind of movie whose opening scene is obviously meant to serve as a skeleton key for the rest of the story to come, but this decades-spanning drama — a lyrical and probing adaptation of David Chariandy’s novel about two siblings coming of age under the care of their Trinadadian single mother in the suburbs of Toronto — is so unstuck in time and shot through with raw emotion that its clunkier moments tend to function like tender maps back to the heart of the matter.