I tend to let Sandy use Defense! because she will cast Haste every now and then which can be a big help.
Tell Mindy to Fight! or Go, go! If you don't have another choice tell her to Do as you will! (assuming that Cindy isn't already reflected). As a general rule, don't let Cindy get reflected.
Cindy is strong, but tell her to Help each other! She will use Auto-Life (or Full-Life if one of the sisters is dead).