Several years ago I foolishly loaned a few thousand $ to a friend of a friend without getting anything in writing. He had agreed to pay back in instalments but very soon stopped making payments. It went a few years without him doing anything to pay me back, and I had no legal recourse and no contact details.
I came to a point where I really could use the money. I found his mother's page on Facebook, and left a polite but specific message, which got the wheels moving. I felt a bit petty and underhanded doing that. But I got my money back.
Though this isn't quite the same situation, if all else fails, as a last resort it may be possible to appeal to your sister's sense of pride to shame her into paying you back. This doesn't mean telling as many people as possible, and certainly doesn't condone slander.
Unless you really need the money though, it's probably better to just leave it alone.