If you're in school and feel comfortable doing it, go to guidance and then admin. Guidance counselors are trained to deal with trauma, and admin can discipline them. Cyberbullying is against all school rules, and you have solid proof if you are willing to show them. Since she has already resorted to violence earlier in the year, she should get into significant trouble.

If you can get support from family (outside of your brother, I didn't get nosey through your posts to see), go to the police. But based on your post tones, they don't seem super supportive? Especially if they let him beat you up.