How could the brother and sister not know (spoiler).....

It was not a mask until just prior to the events of the film. It had taken their mother seven years to use necromancy to draw out the legacy of vampirism from her bloodline. When the mask crumpled at the end, indicated it was not a normal creation, either, but tied to her powers--just like when her mesmerism ceased the moment she was slain. She had also probably been mesmerizing her children for some time, as necessary.

My opinion was it was a mask. The 7 years pertained to her reviving her husband, not herself. The mask was unnoticed because the sister says specificaly she loaths to look at her mother, and does not at All. The son probably barely did. The mask was to. Hide her unnatural youth. In the carriage and in her bed I kept thinking THAT LOOKS LIKE A MASK . DUMB MOVIE HAHA. But at the the end I discover, IT WAS MEANT TO.