Get out of your relationship ASAP. Date other people and let at least six months go by.
Once your ex has a new serious guy, make sure that when you see them, you’re super-friendly -- especially with him.
Now, back to the ex-girlfriend’s sister: Send the “interested” vibe out to her (running into her seemingly by coincidence, choreographing seating so you are next to her, etc.) but do not make any moves. If this is going to work without imploding, she has to ask you out -- that way, you don’t look like a troublemaker.
Sounds like a lot of work? It is. My suggestion is to consider dating someone that just reminds you of her sister instead.