Complex sentence: The Dukes of Schleswig-Holstein-Sonderburg-Glücksburg constitute the senior male line of the family, hold headship by primogeniture of both the House of Glücksburg and the entire House of Oldenburg.
Split into simpler sentences: The Dukes of Schleswig-Holstein-Sonderburg-Glücksburg constitute the senior male line of the family. They hold the headship by primogeniture of both the House of Glücksburg and the entire House of Oldenburg.