Complex sentence: Alexander and Edith's marriage was in difficulties: but the pair agreed to adopt Peggy Piddock, born in 1918, the daughter of Edith's younger sister May, and they were both devoted to their daughter, which likely kept the couple together.
Split into simpler sentences: Alexander and Edith's marriage was in difficulties: but the pair agreed to adopt Peggy Piddock, born in 1918, the daughter of Edith's younger sister May. They were both devoted to their daughter, which likely kept the couple together.