My family keep going through my stuff and taking things. What should I do?

You love your sister I know, but the best thing you can do is to plan to move out. Get a job as soon as you can, retrain if necessary, study for more qualifications if necessary and in the meantime get a lockable cupboard, chest or box to protect your precious items. Your mother takes your sisters side so protect your own stuff as best you can. If you have your own room, get a lock on the door and make sure you use it. Sit your mother down quietly and calmly and explain why this was necessary. No need to fight about it, just look after yourself.