One could argue that the final end of royal marriages as a geopolitical tool ended with the First World War. If there was any sign that royal marriages did not stop wars or led to alliances it was the First World War, where you had the nations of three royal cousins fighting each other, King George V, Kaiser Wilhelm II and Tsar Nicolas II. The war also saw the house of Hohenzollern fighting each other, with king Ferdinand I of Romania from a branch of the Hohenzollern family joining the allies against the central powers, and thus against another member of the Hohenzollern family Kaiser Wilheilm II. In addition to this, most of the royals on both sides of the war were to some degree cousins of each other. Thus, it is clear that by WW1 politics came before royal marriages.

The last geopolitical significant royal marriage I can think of is the royal marriage of Prince Carl of Denmark with Princess Maud of Wales in 1896. The marriage did not carry geopolitical weight immediately, but became of importance about a decade later when Norway became independent of Sweden in 1905. The recently independent nation wanted to secure its independence by forging ties with a stronger nation, and thus decided to adopt a King with ties to the United Kingdom. Prince Carl, being married to a member of the British royal family, suited this goal, and he was elected King of Norway as King Haakon VII of Norway in 1905.