In The Lives of Christopher Chant, Charles, Alice, and Caroline live in a big house in Surrey. Caroline is three years younger than Christopher, and has a fat pony.
In Charmed Life, we learn that she married her cousin Francis, despite opposition from their family. After they eloped to Wolvercote, they were shunned and disinherited by their family. Twelve years into their marriage, Caroline and Francis were killed in the Saucy Nancy Disaster (thus, presumably, disproving the old superstition that witches cannot drown), leaving Gwendolen and Eric orphans.