I don't know much about sisters, but I have played Starcraft: Brood War, and when I picture a power armored apothecary, the Medic comes to mind. So here is some advice (don't take it if you don't want it.) Get a Normal Space Marine body, and replace the head with a SOB one. Make some robes out of putty or Green Stuff (shouldn't be that hard) and attach it to the model below the belt (if you look at the model, you will know what I am talking about.)
Then paint it white, add some Red Crosses here or there, some gold in little parts, and give her a syringe or some other little accessiories.
My 2 cents for free.