She doesn't know that Edmund knows yet, though, and then think about how she approached it when Henry ran off with Maria. She still tried to play it off as not all that bad, and was genuinely shocked at Edmund's reaction. Mary, despite knowing that they don't see the world the way she does, still persists in believing that they will--that her point of view is so reasonable and in accord with the way "the world" sees things, that no really reasonable person could disagree. Neither Mary nor Henry entirely understand Fanny and Edmund yet, for all that they love them. They don't know how deep their principles run.
