Certainly her family will have some say, but if in her turn she wants the title she will have 'right of first refusal', so to speak. her grandfather the present duke can either keep it until he dies, or cede it, but only to the heir-apparent, the present Earl of Mornington. He, in his turn,
must either keep it to his death, or cede it to Lady Mae. A post up there somewhere seemed to say that the present holder could cede it to Lady Mae's brother, who will, in his turn, inherit the remainder of the titles, but this is wrong. The ducado and the title of grandee must go to the direct lawful heir, ultimately Lady Mae, unless she agrees to pass it on, or does not apply to succeed within he specified time. in which case the next heir-presumptive could apply.
I suspect that she will likely pass it back to her brother, but may apply to succeed, enjoy being the Duchess, and then, like Lady Anne, cede it.
Since all of the titles are essentially a memorial to the Iron Duke, I do suspect that family pride will keep them together.