Complex sentence: Coincidentally this was exactly the same age at which his elder sister, Princess Louise, Dowager Duchess of Argyll died, making them jointly the two longest-lived of Queen Victoria and Prince Albert's nine children.
Split into simpler sentences: Coincidentally this was exactly the same age at which his elder sister, Princess Louise, Dowager Duchess of Argyll died. Thereby, making them jointly the two longest-lived of Queen Victoria and Prince Albert's nine children.