You get more experience killing them both (and their creations). One sister is level 16 and the other 20, so even with your creature as a distraction it can be a challenging fight, depending on your level. If you save first you can try it different ways (I sided with the older sister first, got a bit of experience and loot, then reloaded and said I was the strongest).
With the wind gone, you are able to light the three candles around the tombstone.