She “gets” more than enough. So, the money is basically handed over to her then, right? It’s not like she earned it. Maybe that’s part of the problem. If your parents and you just “give” her everything…she may even think that she’s entitled to taking anything she wants.
Has your sister only been stealing from you? Do the two of you get along? Is it possible that she feels insecure/jealous of you…or has some hostility toward you? Kleptomania? Keep in mind that you’re still not 100% sure that your sister took the bangles…but prior to losing the jewelry…if your sister’s behavior had been frustrating you…then maybe you and your parents need to sit down and talk to her.
Maybe either stop giving her an allowance or reduce it (your parents will have to be firm and if they’re not…therein lies the root of the problem)…and tell her to get a job. Maybe when she sees that your parents won’t give her any money to buy the things she wants…it’ll drive her mad enough to get a job.
Also, be sensible. If your sister has a bad habit of just taking things without permission and lying about it…then don’t leave your things out there for her to steal (jewelry, money, credit/debit cards, cell phone, etc). This goes for your parents as well. If she’s got an issue, making it easy for her to steal…and giving her whatever she wants whenever she wants it…is not going to help.