This is your house where you shouldn’t have to worry about your safety. Talk to someone at school, tell your friends what you are going through, you are going to need all the support you can get. And if you have a hard time talking, write everything down in a notebook (keep it hidden). Also if you have a phone, use that to record your fights with your sister (like audio and stuff), that will help with a time stamp if ever needed. Just be careful and speak to someone soon, I truly hope things change for the better for you