It was an idea that popped into my head when you mentioned that they were close in age. ^.^ Half sister technically. I did not notice that either, haha! Seems like them being sisters would be perfect!
Maybe they started realizing that while their powers were similar, they were also very different. So maybe they decided to each go seek out their fathers and that is what lead them down their paths?
I guess it depends on how they separated. Were they on good terms or did it end badly when they went on their way. Personally, I would think it would be better if they did not know.