Become an adult. Instead of texting his sister in hopes that she will tell him about your crush, why don't you try calling him and talk to him.
Of course, if you want to play games, just send him a text pretending that you think you are talking to his sister and tell "her" about your crush. He will get the text by mistake
and your problem will be solved. He will know how you feel and you will avoid having to initiate any direct conversation with him.
Or you could just grow up and stop the games as I said at the beginning. Your call.