Complex sentence: Casimir also signed an alliance with Yaroslav I the Wise, the Prince of Kievan Rus', who was sealed by Casimir's marriage with Yaroslav's sister, Maria Dobronega.
Split into simpler sentences: Casimir also signed an alliance with Yaroslav I the Wise, the Prince of Kievan Rus'. The alliance was sealed by Casimir's marriage with Yaroslav's sister, Maria Dobronega.