This was just the first step in her plan. She missed when all of her girls were little babies that she could take care of, so she's doing something about it. She had slipped your sister some laxatives to make her mess herself first, if she had done all of you at once you might have gotten suspicious.
Before she left, she gave each of you a sandwich to eat. "Eat up!" she said cheerfully. Little did you know, that she slipped a laxative into one of the sandwichs. Who is she now targeting?