Complex sentence: Kujō Yoritsune is also known as Fujiwara no Yoritsune, the Kujō family having been one of the five branches of the historically powerful Fujiwara clan of courtiers.
Split into simpler sentences: Kujō Yoritsune is also known as Fujiwara no Yoritsune because he was a member of the great Fujiwara clan. The Kujō family was one of the five branches of the historically powerful Fujiwara clan of courtiers.