Complex sentence: Over time, the Western Balts consolidated into the Old Prussian nation, while the Eastern Balts of the area, including the Curonians, consolidated into (a part of) the Latvian and Lithuanian nations.
Split into simpler sentences: Over time, the Western Balts consolidated into the Old Prussian nation. The Eastern Balts of the area, including the Curonians, consolidated into (a part of) the Latvian and Lithuanian nations.