I feel the old lady always knew about the stealing but let them do it anyway because she was the only one who ever supported their art career.
I thought the same, the granny gives the impression of the typical wise old character that knows all that happens but don't judge at first, just give a lesson of morality to the main characters in the perfect moment.
I also think that they learned the lesson although the women dying, she gave it to them, but in the hardest way.