I'd say leave it there – ignore him. He could be lying in order to get a reaction from you, which is a very devious plan – who on earth tries to make sisters rivals? If he really does like your sister, he shouldn't involve you. It's heartless! If he approaches your sister, hopefully she'll see what a jerk he is and tell him to hit the road. In the meantime, try to move on – this guy is so not worth your time. Go out, have fun and enjoy getting to know new people!