Complex sentence: His case may have been undermined as well when he came to the defense of his nephew, who had raised a toast to the Catholic James Stuart, who had tried to take the British throne during a rebellion in 1715.
Split into simpler sentences: His case may have been undermined as well when he came to the defense of his nephew, who had raised a toast to the Catholic James Stuart. Stuart had tried to take the British throne during a rebellion in 1715.