Complex sentence: Instead, the adviser told the chancellor that it was in their interest to allow Qin to grow in power, to maintain the balance of power to both Han and Wei so they would still rely heavily on Qi, the most powerful of the three states.
Split into simpler sentences: Instead, the adviser told the chancellor that it was in the interest of Qi to allow Qin to grow in power. This would maintain the balance of power against Han and Wei so they would still rely heavily on Qi, the most powerful of the three states.