Can i obtain a copy of my deceased sister son's birth certificate in Chicago Illinois?

Under Illinois State law, you can obtain a certified copy of your own birth certificate if of legal age and/or the birth certificate of your child. Court-appointed legal guardians may obtain birth certificates of the children under their care if they submit a certified copy of the appointment. Certified copies of appointments can be obtained from the Cook County Clerk of the Circuit Court. Requests made with legal guardianship papers must be made in person or through the mail.