i don't think rewarding either of them is a good idea, because if she is genuinely really scared then all you're doing is setting her up to fail, or if you give her sister a prize it's like saying "you don't get a treat, because you;'re frightened" which isn't going to help, or if it "works" then all you will have achieved is teaching her that when she is scared then she should pretend not to be in order to get something nice!
would she go to sleep in your room?
i think fernie's idea is good. she is old enough for you to be able to say look, dd, i HAVE to do x/y/z, i promise you i will be back in jhust a moment, you can hear me in the next room
you may have to go into the next room and just make some noise, even talk to her while you are in there, and then go back into her so she knows you will come back.
you can gradually make less noise, go away for longer etc etc
really though, it's obviously something she needs to work through, so work through it with her, it won't be forever