Most Helpful Guy
Look that's kinda funny that the one whom you broke up with was dating your sister until you bounced back in his life and now he is leaving your sister for you so what is the proof that he wouldn't leave you until someone more beautiful than you pops up in his life. And honestly I think he is like a butterfly who just jumps one flower to another when the nectar of flower is finished.
Think of your sister how will she feel that you are dating her ex who was you ex too infront of her eyes well that can ruin your and your sister's relationship. So my opinion is don't date with that guy