I suggest you do two things, the first is speak with her existing legal team re an appeal, and the second is to get a second opinion on that if you have no faith in this legal team. your sister will be very frightened and worried for the first few weeks she is inside and will need to know that everything possible is being done to try to get her out. This is really important, so let her know that you are working tirelessly, other than that it's in the lap of the gods I'm afraid, but section 18 is a pretty hefty charge.