The Designee
Until Mona Mayfair, the Designee of the Mayfair family has always been a daughter of the current Designee, regardless of whether or not she is powerful. The process of choosing the next heir is as follows. 1. If any daughters of the current designee survive her, one of them must be chosen. 2. Of those daughters, the one to whom Lasher attaches himself is selected. Mary-Beth commented on this once saying that her eldest daughter by Daniel McIntyre, Carlotta, was not the "chosen one" Because it was Stella who had seen "The Man." 3. The designee cannot be feeble minded unless no other females siblings survive their mother. (As with Belle Mayfair, who was Mary-Beth's true firstborn, by her Uncle/Father Julien.)
Upon the creation of Mayfair and Mayfair, the law firm founded by Julien and his sons Cortland, Barclay, and Garland, and after the passing of Mary Beth, the Legacy came under the control of the law firm and the descendants of Julien as Stella's behest, who had no interest in it whatsoever as long as her escapades could be funded.
Read more about this topic: Lives Of The Mayfair Witches