Final answer:
The indirect object pronoun in the sentence is 'leur'. It is used to refer to 'à vos grand-parents' and indicates to whom the letter is being written.
Step-by-step explanation:
The indirect object pronoun in the sentence is 'leur'.
Oui. Nous leur écrivons.
The pronoun 'leur' is used to refer to the grandparents in the sentence. It replaces the phrase 'à vos grand-parents' and indicates to whom the letter is being written.
Learn more about French pronouns