you sound like an AFC with one-itis!
But if you're still insistant in this sort of complex case and you want to get her however you need to run indirect game on her because the girl has already listened to your step-sister.
This is one of those rare istances where I would recommend indirect (M3 Model) Mystery Method techniques over direct methods. If you can run solid game on her you shouldn't have a problem; girls are easy and will defy every social protocol if they want you enough.
I don't know how new you are to the community but it sounds like this is one of those unique cases where you need the M3 model:
http://ladderwiki.com/w/index.php/M3_Model
- Chris
