Complex sentence: Ricberht was ousted by his Christian rival Sigeberht, who had spent time in the Frankish court; he was established as the rightful heir to the throne, with Frankish help.
Split into simpler sentences: Ricberht was ousted by his Christian rival Sigeberht, who had spent time in the Frankish court. He was established as the rightful heir to the throne, with Frankish help.