Unsure what to do, help?

Ok ladies (gents feel free to reply..) Anyway, despite my commitment issues i'm kind of with/seeing a girl. I have since early February. On the 13th March I was asked to come to a gig to fill in for someone who dropped out. I agreed, despite it not being my kind of music. Anyway, my friend her friends and her sister was there. I've seen her sister around on Facebook and always thought she was cute, the most beautiful eyes, smile and look about her. We get on really well and all I found myself doing that night was looking her her beautiful blue eyes and smiling each time she smiled. I've met her once but I really like her. We seem to get on, I was wondering about asking her out for a drink. But I don't want to make it so obvious that I like her. Her sister knows I like her, but she seems to be staying out of it.

Should I get to know her a bit before asking her out for a drink or get to know her a bit on Facebook then ask to come for a drink with me?

You should get to know a bit before asking her out for a drink. Don't do it with facebook, because it is only build a "fake" comfortability between you two.
Talk to her, and get to know her in real life is better.