The introduction to the Original Penguin Classics edition, by Tony Tanner, discusses the importance of secrecy in S&S. He doesn't say much about Margaret's seeming inability to keep secrets, but BarbarB's point really ties into this idea.

It seems there are characters who can keep secrets, and some whose inabilility to do so is crucial to the plot--and Margaret is sort of treading the line between these two categories. The news about the lock of hair tumbles out, but she's goaded into a revelation about 'F'.

This notion of secrets really has me thinking about this novel in a new way. I'm guessing most of you have already read it, but it was an interesting revelation to me--I wouldn't have read it if it weren't for this GR!