By now, in the present day and age, the concept of being "related" to all the royal houses in Europe is really stretching it. So and so's great grandmother was Queen Victoria just like the other so and so's is. To be honest, how many people count their great grandmother's other descendants children as "family"?
The interactions between the House of Windsor and European royal houses now are basically for political reasons or international diplomacy more than family visiting family back and forth. I agree with you, PetticoatLane, that the first and foremost interactions should pertain to "home" as in the UK and Commonwealth rather than the royal houses of Europe that may have a very distant connection family wise.
Don't forget Brexit.