Complex sentence: His mother, who was 19th Countess of Sutherland in her own right died in 1839 and so her ancient Scottish title passed to George who also became 20th Earl of Sutherland and these two titles became united in the same person until 1963.
Split into simpler sentences: His mother, who was 19th Countess of Sutherland in her own right, died in 1839, and so her ancient Scottish title passed to George, who also became 20th Earl of Sutherland. As a result, the two titles became united in the same person until 1963.