Complex sentence: However, the idea of Biron wielding power was not acceptable either to Ivan's parents or to most of the nobility: during his years as Anna's lover, he had made many enemies, and was tremendously unpopular at court.
Split into simpler sentences: However, the idea of Biron wielding power was not acceptable either to Ivan's parents or to most of the nobility. During his years as Anna's lover he had made many enemies, and was tremendously unpopular at court.