Complex sentence: Nicholas was heir to the title Marquess of Bristol from his elder half-brother John, the 7th Marquess, the only child of his father's first marriage, who married once, in spite of being widely considered homosexual, and never had children.
Split into simpler sentences: Nicholas was heir to the title Marquess of Bristol from his elder half-brother John, the 7th Marquess, the only child of his father's first marriage. The 7th Marquess married once, in spite of being widely considered homosexual, but never had children.