I jointly own a property with my sister following our mother’s death. If we want to draw up a co-ownership agreement, do we need to go to a notary?

Yes. Any co-ownership agreement governing the management arrangements for jointly owned real property must be executed as a notarised deed. Indeed, to make it enforceable against third parties, the agreement is subject to land registration formalities (French Civil Code, Arts. 815-1, 1873-2 and 710-1, para. 1).