i dont understand why it would be akward, ive seen couples where one person is best friends with the other person's sister/brother. anyway you might want to try asking him out again, and tell him that there is no reason why your friendship with his sister should get in the way of your having a relationship. just be careful because if he thinks your asking too much, it might screw things up.... also, how do you know he's attracted to you? coming from a guy's point of view, if i like a girl, and she asked me out, i wouldnt turn her down. you're prob not going to want to hear this but he might not be attracted to you bc if he was he prob wouldnt have said no.
anyway hope i helped and good luck!