Complex sentence: Since he is also without a royal hostess, Mary's sister Princess Anne is summoned back to court (having been banished after an unseemly row with the queen) as his official heiress.
Split into simpler sentences: Since he is also without a royal hostess, Mary's sister Princess Anne is summoned back to court. Having been banished after an unseemly row with the queen, she is now nominated as the official heiress.