Can anyone think of any others?
Count Carl Israel Douglas married countess Louise von Langenstein und Gondelsheim, daugther of grand duke Wilhelm of Baden and Catharina Werner. Maybe not really royal, but still...
Also count Axel Douglas was firstly married to princess Johanna of Sayn-Wittgenstein-Hohenstein. Not really royal either, but still (again)...
The great grandfather of countess Elizabeth Douglas who married duke Max in Bavaria was prince Philipp zu Eulenburg und Hertefeld, by the way.