I see Narcissa shooting daggers at Lucius when she finds out what he did, and he is lucky to be in Azkaban, because otherwise he'd be dead.

I imagine that Pansy (and Blaise, to a certain extent) fill Narcissa in on Harry's role, trying to reach her and staying by Draco's side, so she accepts him more readily in to her son's life than she would have otherwise.

I know I won't have time to write this with everything else, but that's how I see it. She had no idea anything was wrong, but as soon as Lucius gets captured (officially), she runs to Draco's side.