Following the death of our grandfather, my brother and cousins and I have joint ownership of a house. My brother wants to sell his share to one of the cousins without offering it to me first. Does he have the right to do so?

Yes. Article 815-14 of the French Civil Code establishes a right of priority for co-owners only in the event that one of them wishes to sell his or her rights to a third party. In your case, your brother is selling his share to another co-owner. Therefore, you have no priority in this transaction.