Hmm, this reminds me of something I read about in a book called 'how to deal with difficult people.' Some people really hate disappointing other people, so they aren't up front about things. The example in the book was of a guy who told a salesman that he would place an order because he wanted the salesman to like him, but who canceled the order at the last minute.

So I can see how lying about stealing your stuff fits in with your sister's 'selfless' personality. She doesn't want other people to dislike her, so she isn't honest about her bad behaviors.