If your have DPS just kill the closest one.

I run speed runs, we use 1 tank 1 healer and the DDs will just get ready to attack the sister that spawns directly on top of you, pwn that, you can kill it before Garuda/other Sister has even made it to the party lol. Then you kill other.

Otherwise, I would recommend telling your part of the situation and informing them to mark the one they are fighting, so you can off tank the one they're not fighting.

Or SE could help out by putting in colorblind mode, but I dont think they have plans to.