Complex sentence: This was the first occasion that a Summer Olympics held in a Commonwealth realm was not opened by the monarch or a member of the Royal Family, and the first Summer Olympics since Melbourne in 1956 not to be opened by the head of state.
Split into simpler sentences: This was the first occasion that a Summer Olympics held in a Commonwealth nation was not opened by the monarch or member of the Royal Family. It became the first Olympics since Melbourne in 1956 not to be opened by the head of state.