Note: Frank was a descendant of the kings of Avenwell. His nearest royal relative was his grandmother, Princess Sarah Pearl Latham Rundangh.
When he married Mary Cooke, King George gave them the lands and Latham Castle on Rundangh Island. (This was the inheritance that his mother, Nellie, refused.) He also gave them their titles, the first Duke and Duchess of Rundangh.
Upon the death of his parents and grandparents, Frank Avenwell would be the sole heir to the Avenwell side of the families' fortune, making him one of the wealthiest men in the world.
And at his death, his entire fortune was left to his wife, Mary, who had already inherited 1/4 of the Cooke side of the families' fortune, making her one of the wealthiest women in the world.