This contains SPOILERS for recent Batman comics (Batman #10).
In Batman #10 by Scott Snyder and Greg Capullo, it is revealed that mayoral candidate Lincoln March is both a member of the Court of Owls and also, allegedly, Batman's younger brother (Thomas Wayne Jr.). The backup story in the issue shows a flashback decades in the past, when Bruce Wayne was only 3 years old. The flashback shows events that Lincoln March referred to in the main story. When Bruce was 3, his mother was in a car accident while pregnant with Thomas Wayne Jr. (the accident occurred at the corner of Lincoln St. and March St. to boot!). He was then born early and put into a childcare facility where he remained after his parents were murdered. Therefore, Bruce never knew he had a brother.
Does any of this exist prior to the New 52? Does it exist post-Crisis, or is this all Golden and Silver age mythology?