Ответ:

  1. My aunt’s name is Julia. The possessive form is needed.
  2. Have you got a little brother? In a question, put “have” before the subject.
  3. My friends’ bags are in the classroom. The bags belong to the friends.
  4. My sister hasn’t got a favourite singer.
  5. Whose is this girl in your grandma’s garden? “Whose” asks who someone belongs to or is related to.
  6. Come and look. Those are my new books. This sentence is already correct.