Who is Sabine de Kercoz from Possession: A Romance and what is their importance?

Sabine de Kercoz is a distant cousin of Christabel LaMotte. She is an aspiring poet and novelist and writes extensively in her journals and letters about Christabel's visit to her family's home in 1859. She is 20 years old, unmarried, and living with her father when Christabel comes to their home in 1859. She provides important information in her journal confirming that Christabel was pregnant in 1859, and that is why she stayed with them.