Not least Henry's own marriage to his cousin and former sister-in-law Catherine, and the marriages of her two sisters to their another cousin, King Manuel (who married a niece of his first two wives as his third wife). As well as the marriages of Henry Tudor and Elizabeth of York, also cousins, and Ferdinand and Isabella, also cousins (actually all four were cousins). Not forgetting Eleanor of Aquitaine was Henry II's cousin and the former lover of his father, or that their children Joanna and John also married cousins.
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At the time of Philippa of Clarence's marriage no one was looking to her to inherit the throne. The Black Prince had just married his close cousin, the notably fertile Joan of Kent and begun his family (having already fathered a number of bastards). Philippa's own father was still a young man, travelling to Italy to fetch a new bride certain to give him some more heirs. Certainly no one could have foreseen a Lancastrian usurpation at that point.
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The existence of a prince Edmund/Edward is an interesting PoD. I don't see him marrying one of his brother's subjects, nor do I see him making a grand foreign alliance - at the time of his marriage it would no doubt be expected that his dashing elder brother would father plenty of heirs from the dowager princess of Wales.
I would venture an initial betrothal with Claude of France, instigated by Queen Anne and perhaps with a view of securing Brittany's continued independence; a betrothal no doubt dissolved and replaced once it became apparent Anne would have no living son. Henry VIII might seek a pro-Hapsburg match for his younger brother, for example a Portuguese infanta; once he became disillusioned with Ferdinand and Charles, a switch to Marguerite d'Angoulême (sister of the future King of France) might also make sense. If he was still unmarried by the mid-1520s, I can see Henry marrying him to Mary and securing the succession that way.