Complex sentence: His rival, Somerset, belonged to the Beaufort family who, like Henry, were descended from John of Gaunt, Edward III's fourth son, though by his former mistress, Katherine Swynford.
Split into simpler sentences: His rival, Somerset, belonged to the Beaufort family. Like Henry, they were descended from John of Gaunt, Edward III's fourth son, though by his former mistress, Katherine Swynford.