or does JKR's answer to the FAQ "What happens to a secret when the Secret Keeper dies?" portray Sirius as...for want of a better adjective...'weak'? It's official now: the secret cannot be spread further once the secret keeper dies - not even the Potters could have divulged it had their secret keeper died. Which means that, had Sirius not passed on the responsibility to Peter, he could have taken the location of their hiding place in Godric's Hollow to the grave and they would never have been found by Voldermort - assuming of course, that he was caught (and later interrogated by means of veritaserum, which, also according to Rowling, is not infallible). Couldn't measures have been taken to ensure that Sirius would never be captured alive, and a contingency plan to silence himself somehow in case that did happen?
And why am I wasting time on yet another of JRK's inconsistencies?
edit: I haven't learnt my lesson. With regards to that interesting theory I read a couple of months ago, JRK said this. Not that I didn't actually have issues with the theory (someone knowing everything that happened could change the future!) but meh, it's an amusing and interesting theory nevertheless.