How do I go about liking my homeboys sister??

Ok so I have a homeboy who is 21, I'm 21, and his sister is 20 years old. We started liking each other me and his sister and she told me one day when we were outside talking outside my car that she liked me, and it took me by surprise at first but I was glad that she said it because I was too afraid to tell her that I liked her. Now that we like each other and it's out we talk a lot more and it's just great I really care about her. But the thing is I never expected to fall for my homeboys little sister because he's like a brother to me and we also spend a lot of time hanging out with friends. I don't think he knows that me and his sister are talking to each other the way that we are but I really like her and I don't want this to mess up or make him upset. What should I do this has never happened to me before and it's harder because I just really like her.