Complex sentence: His sister was Iris Mary Butler (was born in 1905), who became Iris Portal upon her marriage, and whose elder daughter is Jane Williams, Baroness Williams of Elvel, the mother of Justin Welby.
Split into simpler sentences: His sister was Iris Mary Butler (was born in 1905), who became Iris Portal upon her marriage. Her elder daughter is Jane Williams, Baroness Williams of Elvel, the mother of Justin Welby.