Continuity mistake: When Corrine takes off her blouse, she shows her children seventeen scars on her back that she got from being whipped by the grandmother. Later in the movie, when Corrine is dancing at a party, she is wearing a strapless dress and there are no scars or any signs that she was struck with a whip.
Other mistake: When Christopher is in the bathroom stupidly "dabbing" Cathy's back as she's taking a bath there is a glass block window behind them. The window makes no sense. It doesn't match the era or architecture of the bathroom or the home, first of all, but probably more importantly - what's behind it? Where is the light coming from? It's just this random, standing glass block window with an equally-random light source. (00:34:53 - 00:56:49)
Answer: When the children discover the cookies had been poisoned (revealed by the death of their pet mouse which ate part of one of the cookies), they suspect their grandmother. After all, she was the one who had been cruel to them all along. But eventually, they realise that it was in fact, their mother, Corinne, who was lacing the cookies with poison. She was doing so with the hope that the children would eventually die, seemingly from disease. In doing so, she would secure the family inheritance which was contingent upon her never having had children from her first marriage. They eventually confront their mother on the day of her second wedding, and a struggle ensues which ultimately leads to Corinne falling to her death from a balcony.
Michael Albert