Following the death of our parents, we signed an act of sharing with my sisters. Two days later, one of them came to the family home to claim a property that had been assigned to me. Does she have the right to do so?

No. Following the partition, each co-heir is deemed to have succeeded alone and immediately to all the effects included in his lot and cannot take ownership of the other assets of the succession (art. 883 C. civ.). Consequently, it is not possible for a co-partitioner to appropriate property attributed to someone other than himself.