Hmmm, tricky one! So did she actually speak to your sister about you then? Would your sister tell you what was said?
This girl may just be unsure about whether you actually like her or not...she could just be being cautious so she doesn't make a plonker of herself by making any move and getting knocked back.
Like you guys, us girlies don't enjoy knockbacks and it can be hard to make the first move!
Unless you have something to worry about or hide (embarassing past etc) then I can't see why it would be a bad thing that she might talk to your sister about you.
The way I see it, you can go down one of three routes....

Sit and wait to see what happens and what the conversation with your sister actually involves...maybe wait for another 'friend in common' night-out pops up.

Send her another message on bebo 'Casual' to see if it gets any sort of ball rolling. Although this could take time if casual messages are just going back and forth and it may not give her any real confidence that you really do like her.

Contact her and just put it straight....For example "I am sorry if this seems a little forward, but I'd love to take you out for a drink and get to know you a little better. How does next weekend suit you?"
The 3rd option would get the quickest answer with less faffing about, however you may have to be prepared to get the 'dreaded knockback' or no response....but at least you'd know either way.
Whichever choice you make, you're gonna get the same answer at the end so if it was me, i'd go for the short, sharp resolve.
Girls like a guy with confidence but not arrogance. Get the balance right and you'll do well. Getting that 1st proper date isn't easy, and it doesn't get any easier as time goes on but it's all kinda what the fun is all about and keeps it interesting!
Hope it goes ok for you anyway....let us know how you get on!
Yours faithfully, Auntie Charlie!