Complex sentence: A half-brother, Prince Vajiravudh (as the next eldest) replaced Prince Vajirunhis as the Crown Prince he eventually succeeded to the throne in 1910 as King Rama VI.
Split into simpler sentences: A half-brother, Prince Vajiravudh (as the next eldest) replaced Prince Vajirunhis as the crown prince. He eventually succeeded to the throne in 1910 as King Rama VI.