Your sister needs serious help. If she isn't taught now to stop she'll get caught and go to jail because it will go beyond stealing from home. I had a problem once with someone very close to me. They were much younger but I caught them shoplifting. I made them take the item back, put it on the counter then I returned something I had bought for them from that same store. Then the next day or so I took them to talk to our minister. He talked to this person by them self which was very intimidating. She needs an authority figure. You and your Mom just need to flat out call her out on it, let her know you know she's stealing and you won't tolerate it because it's wrong. I really hope this helps. She's definitely headed down a bad road. Good Luck.