Bonjour
1- This is Harry and Ron's idea, its their sister. (pluriel = leur)
2- This is Hermione's cat : it's her cat. (féminin)
3- I am Harry Potter. Hermione and ron are my friends. (mes amis)
4- We are Harry and Ron. Hermione is our friend. (notre amie)
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bonne journée ☺☺☺