So what's the story with your sister? is there a chance for reconciliation or are they gonna cat fight if they see each other?
I'd say you go for broke...
"Hi Jezebel, My sister would KILL me if she knew I was reaching out to you... This is Chris. "
If she replies then read her mood. Maybe she wants to spite your sister by deflowering her little brother, maybe she wants to reconcile...you need to get a read on that and exploit it. WITHOUT asking directly....
If it is revenge, then work the illicit affair angle...Think Romeo and Juliet.
Reconcile, then "let's meet up and I can give you some ideas on how to mend this relationship..."
Worst case scenario she blocks your number and you never talk to her....Won't know till you try...