If that refers to Harlequin as Earl of Arran, he inherited those lands from Aina, who had purchased "the defunct earldom sometimes around the turn of the 21st century."
So in addition to the House of Lords Act of 1999 not passed, it would require the Earl of Arran title to go extinct (which it currently isn't, being a subsidiary title of Duke of Hamilton), then Aina purchase to involve not only the land, but also a peerage created by the King or the Queen of the United Kingdom, and this peerage to have a letters patent that allow the title to pass to mere beneficiary of the estate rather than only legitimate male descendants as the norm is (unless Aina had prepared documents establishing Harlequin as her legitimate son, that is).
In real life, the castle and land of Arran were bought in 1995 by Swiss millionaire Willi Ernst Sturzenegger. He petitioned to claim the title of "Feudal Earl of Arran". He was not claiming the peerage (the difference between "feudal earl" and "earl") that belonged to the Duke of Hamilton. Anyway, he did not get his wish granted.