Maybe not. I had almost the exact same thing happen to me a while back (except my wallet likely fell out of my bag and was not actually stolen). The postman who delivered the package told me that someone probably found the wallet and dropped it off at the nearest post office.
Was he kind enough to put his return address on the package, so that your sister could send a thank you note (you know, one that's wrapped in blue)?
When I was a garbage man in Portland, I found a woman's wallet which had her drivers license, credit cards, address book, and personal pictures. I called a couple of the numbers from her address book and finally got in touch with her. She came and picked up her stuff and gave me $40 for my trouble. That was nice because it was Christmas time and I used the money to buy my (then) girlfriend a $250 gore-tex jacket. That girlfriend has been my wife for almost 9 years now. It's all about the Karma.
The story of your sisters experience just reminded me of something. As I pulled into a convenience store I saw something fall off of a car just exiting the parking lot. When I got out of my car I walked over and picked the object up and discovered it was a wallet. When I got home I looked inside and found a phone number and called. A few days later a woman came to my house to pick up the wallet. She quickly looked inside and demanded to know where the $100. dollar bill that she had hidden inside. I did not take ANYTHING out of her wallet so if there had been $100. in there it would STILL have been in there, because no one had touched the wallet but me. I was so angry at her accusation that I told her to get off my $%^&*( 'n porch. From that little incident I realized the truth that no good deed goes unpunished. $%^&*(# b*tch!