Complex sentence: Mithridates also took a part of Galatia that had previously been part of his father's kingdom and intervened in Cappadocia, where his sister Laodice was queen.
Split into simpler sentences: Mithridates also took a part of Galatia that had previously been part of his father's kingdom. Mithridates intervened in Cappadocia, where his sister Laodice was queen.