We are two brothers and one sister co-owning an agricultural land. My sister and I want to lease it, but my brother opposes. Is the agreement of all co-owners required ?

Yes. Regarding co-ownership, Article 815-3 4° of the Civil Code states that a majority of two-thirds of the co-owners can "conclude and renew leases other than those concerning a property for agricultural, commercial, industrial, or artisanal use." In your case, it is a lease for agricultural use, which requires the consent of all co-owners.