If you let Sandy perform Reflect of Cindy, Mindy has a tendency to perform bounce magic off of Cindy rather than use attacks or Passado, which is the real power move of the Magus Sisters. And Sandy usually defaults to using Reflect on Cindy before attacking. So, when you use the Magus Sisters, if possible do not tell Sandy to "Do as you will" or "Defense!". If those are the only two options, go with "Do as you will" and hope for the best. Cindy takes a break usually because if you tell her to "Do as you will" and the girls are fine healthwise then she just doesn't do anything. Also, if there are just airborne enemies Cindy doesn't think to attack. Mindy will usually attack anything, and performs Passado quite often if Sandy doesn't perform Reflect. And, the more you use the Magus Sisters, the more often they will have more options to choose from, in my experience.