Go to Troy Dunn who is called "The Locator" he has a show on tv and a website where you can send him a video explaining your situation and he might help you. I love that show, it always makes me cry seeing people united. Good luck!
The best free site to find lost loved ones is Facebook. You can search for your sister or any other people who may know how to locate her. You can also watch for sites like ancestry.com to have free trials. Good Luck in your search!