Academic misconduct: If a schoolmate directly uses your paper without indicating the source or your authorization, it may be regarded as academic misconduct. If discovered, it may have a negative impact on your academic reputation and future development.

Repetition rate: If you find that your sister's paper has a high repetition rate, your sister's paper may be considered unqualified or need to be revised, and your reputation in school may also be affected.

Legal issues: If the papers used by your school sister involve legal issues such as infringement or plagiarism, you may need to take legal action to protect your rights and interests.

Therefore, it is suggested that you communicate with your junior in time after you find him using your self-taught undergraduate thesis, and ask him to indicate the source or get authorization to avoid possible academic misconduct and legal problems. At the same time, you should also keep relevant evidence and records for future use.