Oh that would be really cool! I hadn't thought that far ahead to sisters or I guess it would be more like step sisters if they had a different mother. What is cool is Grace's third power is kinda close to Elena's pain one so it would kind of work out. 

The fact they went their own ways when they started developing their powers makes sense. It would have started becoming clear which path they were going to take around then.

Which brings another question to mind. If after it being so long, if they know which coven the others in? Because if not that would be on hell of a surprise for the two of them!