What can we do? my sister is stealing...?

ok, so my little sister (age 12) has been stealing my belongings and coming up with these huge lies that contradict themselves. She steals stupid things like play-doh and tell us shes gotton them from her "friend" and look she can prove it because if has her friends name writtin on a piece of tape (that we saw her grab and put there) in her handwriting. once we notice the items are missing she finds subtle ways to put them back that clearly werent supposed to look suspicious (a pile of crystals neatly stacked under my dresser across the room from were they originally were) The stealing has grown to more expensive items like my mothers rings. Its not like she doesnt have any of the stuff i have. shes quite spoiled and pampered. my mom is aware but doesnt know how to deal with the situation and any answers would be appreciated!