Its a done deal.
You will hang out with her in a group. Most likely with her friends. Her friends will ask about you. They will find out you are her sisters ex. They will be all over you. Flirt back but don't take it any farther. It will drive her crazy to see all of her friends all over you.
At some point she will get up to go to the bathroom. Give her a minute then head to the bathroom yourself. When cross her path stop and talk to her. Give her some individual attention. She will want it.
Eventually you guys will go out to dinner or what ever. After a few drinks you guys will get after it. You both will know its going to happen. The drinks are just there for an excuse if needed.
It will be an "under the radar" type relationship for a while. After numerous sex sessions you guys will get careless and get caught.
It will be worth it