I would suggest just having DPS immediately unload on whatever sister pops where you are no matter what color line she is if you go to one of the sides where a sister pops. Getting either sister killed immediately is a huge advantage and it gives the off-tank time to build hate on the other one. Waiting for the "correct" sister to get to the party takes a bit and all the while the timer is ticking before they Mistral Shriek.

Also make sure WHMs keep Shroud of Saints ready for that part as they will likely need to throw out at least 1 Medica.