The houses follow the male lines. If a British prince marries a Spanish princess then it is still the British royal family and not the Spanish. So when they become King and Queen and then die without any living spawn the Spanish King and brother of the late British Queen would still not carry the British royal house on even though they are closely related. More then likely they have to go back a few generations to find a British prince who did not become a King but rather got a Duchy or something in Germany and started their own dynasty.
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