don't worry about it. she's your sister, even if she stole it just think of it as the last gift she'll ever get from you. be sure to hide your valuables in a better spot.
i lost my wallet one time in my car. it happend 2 years ago and still i dont know how i lost it. lost my car keys in my apartment. i was the only parking spot indoors over a drain... one night i think they fell. still the wallet lost trips me out....