Complex sentence: Members of this line bore the title of "Duke of Schleswig-Holstein-Sonderburg-Glücksburg", but had limited powers in ruling their territories, as it was not an estate of the Realm, but subset of the Duchy of Holstein-Gottorp.
Split into simpler sentences: Members of this line bore the title of "Duke of Schleswig-Holstein-Sonderburg-Glücksburg". However, they had limited powers in ruling their territory, since it was not an estate of the Realm, but a fief of the Duchy of Holstein-Gottorp.