Ahh the girl next door question. There is a good chance that if you go after the relationship with his sister that you will end up losing both. It is very easy for you to believe that he mistreats her as you have different interests in the relationship. You may feel like you are doing the best thing, but in the long run you would only be interfering in normal sibling disputes.
After all is said and done they will still be brother and sister. . . . It would be my best advice unless all three of you are of older age, and have more experience with and or in relationships to stay away from this. As the chances of it failing will be high, and your losses will be a friendship and a girlfriend.