HI ESMAN,
Bullying is a terrible experience to go though, maybe could seek out some local support groups for help etc. counselling as well may be helpful, if you felt wanted to try that route. I agree, a lock on your bedroom door to keep your things safe. Not right they are removed/ damaged as causing Obvious distress.
Maybe could try talking to your mum about how your feeling, and how your sister's BEHAVIOUR affects you. Could your dad try talking to your mum and working it out together as a family. It can damage self confidence, so a possibly is trying new hobbies/ interests to gain more confidence and independence.
Take care
Kate.