Most Helpful Guy
Your sister has done you a favor and hopefully simplified the process of you getting a girlfriend. She knows you and convinced someone else she probably knows to go out on this blind date with you.
This date is to have fun and to get to know her. There is no need to compliment her on her looks- focus on making a good impression (incuding paying for the date), ensuring she has a good time, and determining whether you want to go out on another date with her.
Since your sister set you up with her, she should be able to tell you what she likes to do, eat, etc., and what her personality is like. Be yourself, though. Be friendly, but respectful. You usually can't make up your mind about someone after a single date (unless they really make a bad impression), so just take it easy and have a good time.