If all sisters are present they have a chance of using Delta Attack.
Sandy is not too big of a threat, and thus the player should save her for last. One strategy is to use Paine's special dressphere, the Full Throttle's Sword Dance to attack all three of the sisters at once. The Sinistral and Dextral Wings should focus on defense/support until Mindy is defeated, as she has the least HP in the trio, and unless all three sisters are active they won't use the lethal Delta Attack. After Mindy is defeated, the Dextral Wing should switch to offense. Since Cindy is the only one who can heal, she should be the next priority.
If one doesn't want to use a special dressphere, using two Dark Knights and an Alchemist works well, with repeatedly attacking with Darkness and healing by mixing. Once Mindy's down, the player can have one or both of the Dark Knights begin using Black Sky instead of Darkness.
The Magus Sisters can be unlocked as a team in the Fiend Arena by capturing a Jumbo Cactuar and completing its fiend tale. Jumbo Cactuar can be captured during chapter 5, once the final cactuar gatekeeper has been returned to Cactuar Nation. To capture Jumbo Cactuar, the player must place a L pod in Bikanel.