There isn't much you can do. You almost certainly do not have a cause of action for defamation.
To have a successful basis for a defamation suit, the person must be saying or writing a lie about you and they MUST KNOW IT IS A LIE. (And you have to prove they knew it was a lie.) Here, your sister's friend will be able to argue she has a good faith basis to believe you are a thief. That ends any hope you have of a lawsuit.
Even if you could prove she was intentionally lying about you to try to ruin your reputation you would still have to prove damages. Being embarrassed and stressed, and having to defend yourself isn't really what the statute was designed to deal with. It is more about losing your job or losing business. (So, if you were a housekeeper and the accusation that you were a thief caused your clients to fire you, you would have damages.)