I'm with leejosepho. If you don't already have a relationship with this kid, I would think it would be terribly uncomfortable for him to be approached and outed with no warning or prior knowledge about AS. If he gets to know you and see's that he has a lot of traits in common with you, he'll figure it out.
If he's struggling and needs to get help, then maybe he needs a little prodding in the right direction. But, if the sister wants him to know just for the sake of knowing, I would definitely stay away from an open conversation right off the bat. I would have been very uncomfortable and probably would have denied most of my true characteristics to anyone who approached me with words like Autism and Aspergers. For me, it's something personal that I'm happy to have figured out about myself. It's made me more aware of how I act and has helped me to keep some of my characteristics in check when I need to (i.e. at work). On the other hand, I'm now hyper aware of any media coverage, movies, tv shows, etc. concerning autism in general when I'm in the same room with the one person I've ever told. For me, personally, I think it would be worse if that person had realized what I was before I did.
Just tread lightly...and good luck 
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Damn good, bloody good, damn good job.