Okay... the Sister of Battle has a bolter aimed, point blank, at the Banshee's chest, in a lethal spot. Her finger is on the trigger. The bolter fires, if I recall correctly, brutal penetrating shells that explode on entrance into a body. One pull of the trigger and the bolter round goes flying.
Banshee. Has a pair of swords, very nice and likely incredibly deadly swords but still swords. Her right hand is being held back by the Sister, and while the Eldar tend to be faster I don't think them being physically stronger is a part of the lore. Due to the grip, the Sister will be able to keep that hand at bay, for at least long enough to pull the trigger. The question then is the left hand, and the sword it's gripping. While the hand is free, the weapon angle is in a bad place, more likely to hit the pauldron than the gun or arm, much less any vital spots.
The banshee will have to twist her wrist to be able to bring the blade down to strike a significant area that will allow her to continue melee combat or insta-kill the Sister. The Sister has her finger on the trigger and one of the most brutal weapons in the Imperial arsenal point-blank against the Banshee's armor. And that's just assuming one shot as opposed to multiple. Conclusion, the shot is going to hit the Banshee. The Banshee's armor is the saving grace that would let her win. If the shot pierces the armor into her body, then she's dead.