Complex sentence: He married firstly, in 1954, Mary Burke Roche (born 1934), a daughter of the 4th Baron Fermoy and the elder sister of Frances Burke Roche, later Viscountess Althorp and then Frances Shand Kydd, the mother of Diana, Princess of Wales.
Split into simpler sentences: He married firstly, in 1954, Mary Burke Roche (born 1934), a daughter of the 4th Baron Fermoy. Mary was the elder sister of Frances Burke Roche, later Viscountess Althorp and then Frances Shand Kydd, the mother of Diana, Princess of Wales.