We are 3 brothers and sisters who own an undivided piece of farmland. My sister and I want to rent it out, but my brother is against it. Do we all need to agree?

Yes, in principle, if only some of the undivided co-owners wish to enter into a lease on an undivided property, they must hold at least two-thirds of the undivided rights in order to do so. However, this rule does not apply to leases on agricultural real estate (art. 815-3, 4° C. civ.). It is essential to obtain your brother's agreement.