Complex sentence: Izyaslav's children Yaropolk and Sviatopolk would rule the Turov Principality authority of which was mainly challenged by Rostilavichi of Rostislav Vsevolodovich.
Split into simpler sentences: Izyaslav's children Yaropolk and Sviatopolk would rule the Turov Principality. Their authority was mainly challenged by the Rostilavichi of Rostislav Vsevolodovich.