It is important to consult with a solicitor who specializes in privacy or family law. But I’m gonna give a general advice like the extreme case if you decide to call the police and take this to court.
Injunction: You could apply for an injunction, specifically a prohibitory injunction, to prevent your family member from sharing the pictures without your consent. To obtain an injunction, you would need to demonstrate that there is a real and imminent threat of disclosure that would cause you harm or distress. The court will consider factors such as your right to privacy and any potential harm or damage that may arise from the disclosure.
Protection from Harassment Act 1997: If your family member's behavior can be categorized as harassment, you may be able to take legal action under the Protection from Harassment Act 1997. Harassment is defined as engaging in a course of conduct that causes alarm or distress to you. You may consider gathering evidence of the harassment, such as screenshots of threatening messages or witness testimonies, to support your case. You can seek a court order, such as a restraining order, to prevent your family member from engaging in further harassing behavior.
Or carrying a pocket knife to school, it is essential to report this information to the appropriate authorities. Contact the school administration or inform the police so they can address the potential safety concern. Or you can tell your family member to stop sharing pics or she will tell the school about this.
Now don’t take my advice this is just a extreme case cause at the end of the day she is underage and only sharing pics to people who have seen you.