Leur vs Leurs

Question

J'écrirai à mes cousines. Je leur écrirai bientôt.IAs "cousines" is plural, is there a reason for saying leur instead of leurs? Perhaps I have misunderstood the rule.

Accepted Answer

Leur is the plural indirect object pronoun in 3rd person. You may be confusing its use with the possessive adjectives, leur and leurs: 
https://progress.lawlessfrench.com/revision/grammar/when-to-use-lui-and-leur-to-replace-specific-people-with-him-her-and-them-indirect-object-pronouns
 
https://progress.lawlessfrench.com/revision/grammar/use-notre-nos-votre-vos-leur-leurs-to-say-our-your-and-their-possessive-adjectives

Answer

Just to add some examples to Maarten's explanation:
Je leur écrirais. -- I'll write (to) them. Leur is the indirect object pronoun denoting a group of people. It is always leur and never leurs.Je leur donne leurs livres. -- I'm giving them their books. The first leur is again the indirect object pronoun. The second leurs is the possessive pronoun. It has the -s ending because it matches livres (boos), which is plural.Je leur donne leur livre. -- I'm giving them their book. The first leur is again the indirect object pronoun. This time, it is only one book, so the second leur is in singular, without the -s.