Possible Clues:

• Many 'Sister Act' characters
• "The Sound of Music" extras
• Sister's insurance doesn't include Erica
• Convent group
• Joshua's father's sisters
• Some parochial school teachers
• The Three Sisters, perhaps, reviewed by leading newsman in The Star
• Convent managers
• Cloister residents
• "Finale Ultimo" chorus in "The Sound of Music"