My best advice is to wait until the sister leaves and she is there alone. Then, play drunk one night. Come home and pretend you've been drinking, you're all loaded up and want to relax. Take a shower, slip on a robe, or just stay in the towel, and sit on the couch to watch TV. Make sure she can easily see you're naked. Have your cock exposed. Pretend to pass-out. And see what she does!
If she does nothing.....she'll never do anything.
