Porn has taught me that any time two women are in a room together they get it on. So record her and your sister going at it and link me to the video. I promise I won't show gaf.
If you know where she lives why not go over there when she's asleep, break in and cover her kitchen floor with rose petals.
Blood red rose petals.
Then smash the fucking place up and beat a hasty retreat.
She'll wake up in the morning not only startled but also certainly impressed not only by your commitment but also by your burglery/vandalism skills.
She'll either marry you, kill you or have you arrested.
I'd say you'll have about a 33% she'll have sex with you - that's after and if she marries you - and I think those are pretty good odds.
If else fails though, you could always just have a wank.
Let us all know how it goes.