Complex sentence: Metacomet became sachem of the Pokanoket and Grand Sachem of the Wampanoag Confederacy in 1662 after the death of his older brother Grand Sachem Wamsutta (called "Alexander" by the colonists), who had succeeded their father Massasoit (d.1661) as chief.
Split into simpler sentences: Metacomet became sachem of the Pokanoket and Grand Sachem of the Wampanoag Confederacy after the death in 1662 of his older brother, the Grand Sachem Wamsutta (called "Alexander" by the English). The latter had succeeded their father Massasoit (d.1661) as chief.