The Lennox Stewarts and the Hamiltons were rivals, with both claiming to be next in line for the throne in the event of the royal Stewarts going extinct.
Both derived their claim from Mary, a daughter of James II, who married James, 1st Lord Hamilton, and had two kids (who matter for the sake of this scenario):
- Elizabeth, who married Matthew Stewart, 2nd Earl Lennox (-> grandparents of Margaret Douglas' OTL husband John, 4th Earl).
- James, 1st Earl Arran (-> father of James, 2nd Earl Arran/Duke of Chatellherault).
James, 1st Earl Arran had some marital irregularities, and some held that this made his descendants illegitimate.
The dispute had a further layer of bitterness because John, 3rd Earl Lennox had been murdered by John Hamilton of Finnart (illegitimate son of 1st Earl Arran).
Anyway, the result is that a Civil War is not off the table when the royal Stewarts go extinct - depending on when this breaks/how the factions take shape/religious developments in Scotland etc. may preclude a marriage between *King James VI (Hamilton) and a niece of the King of England.