If she did kill her girls, I find it incredibly fascinating that she left her son alive. I can follow the twisted logic of a mother who is in such a dark place that she believes her kids are better off dead with her, but to leave one behind? So I’ve tried to think of a different line of reasoning that would make sense to a mother willing to kill two of her three kids and leave the third unharmed. Perhaps simply because he wasn’t there? If it was a spontaneous, impulsive act, I suppose that could make sense.
I assumed her son was at college so not in the house?