Complex sentence: They are believed to have had a happy marriage, though in the 1860s Caroline's mental and physical health both began to decline and her husband's death in 1874 only succeeded in making her more infirm, eccentric, and temperamental.
Split into simpler sentences: They are believed to have had a happy marriage, though in the 1860s Caroline's mental and physical health began to decline. Her husband's death in 1874 only succeeded in making her more infirm, eccentric, and temperamental.