Complex sentence: His sister, the Honourable Catherine Pakenham, was the wife of the Duke of Wellington, while his younger brother the Honourable Sir Edward Pakenham was a British Army officer who served under Wellington in the Peninsular War.
Split into simpler sentences: His sister, the Honourable Catherine Pakenham, was the wife of the Duke of Wellington. One of his younger brothers was the Honourable Sir Edward Pakenham was a British Army officer who served under Wellington in the Peninsular War.