Complex sentence: Ponsonby was born in London the first son and third child of Edward, 8th Earl of Bessborough and Blanche "née" Guest; her father Sir Josiah Guest was a great-uncle of Sir Winston Churchill.
Split into simpler sentences: Ponsonby was born in London the first son and third child of Edward, 8th Earl of Bessborough, and his wife, Blanche. Her father, Sir Josiah Guest, was a great-uncle of Sir Winston Churchill.