I have never seen a single silent sister instakill the dragon...ever...
It's hard until you learn how to chain up spinning blades and throwing blades. Spinning blades is even harder to chain up on the dragon because of his body size and might need to position yourself before hand. You also need to mix in some hidden blades too because that's a heavy hitter whilst you are hitting ferociously with spin/throwing blades 
You can't afford to miss any of your spinning blades, and it's all about the last few hits just before the dragon dies / take off. Because the last 2 seconds you will dish out more damage than you did for the rest of the fight added together. The last 12 hits or so makes the real difference.
If executed well, the last few hits does approx 40 to 68k per hit and you will 1 shot the dragon when he still has at least 1/3 of its health left. If you miss 1 or 2 seconds, then the damage per hit will drop to about 20k, assuming no other mistakes. Not so good any more huh? Worst thing you can do is break that chain mid way, e.g. missed a spinning blade attack or dragon is out of your spinning blade's reach and you had to move over.
Oh, and dragon definitely won't jump in this scenario...