If this is a legal guardianship - ordered through the courts - it is of public record. However, it may not be available for the public to see. Some states restrict access to guardianship files - especially financial information. Please consult a guardianship attorney where the 16 year old is supposed to reside to discuss options.
In Texas all guardianship proceedings are public records unless ordered sealed by the judge. That would be very unusual. I don't know how old you were when your father died but if you were over the age of 12 you would know if a guardianship had been created. You would have been interviewed by a court investigator and by a person called an attorney ad litem who was appointed by the court to represent you. Plus, if your natural mother is still living and her parental rights have not been terminated then it is impossible for the court to appoint anyone as your guardian. I would guess that your step mother is not being honest with you.
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.