Sophie Bailey (née Sophia LaMotte) was Christabel LaMotte's sister, and Sir George Bailey, Sr. is the man she married. As you may have guessed already, Possession's own shotgun-wielding Sir George Bailey is a direct descendent of the previous Sir George.
As a child, Sophie didn't share much in common with her literary sister. Whereas Christabel loved to listen to their father talk about folk tales, fairy tales, myths, and legends, Sophie wasn't interested. As Christabel writes in one of her letters to Randolph Henry Ash:
"My sister Sophie took no interest in these matters. She liked things women like—pretty things—she was no reader—it irked her, that we lived secluded—as it irked my mother—who had supposed that a Frenchman was always a galant—a Man of the world—or so I believe she supposed—for they were ill-matched." (10.87)
Sophie and her sister may not have shared much in common when they were young, but that didn't stop Sophie from coming through for Christabel when our heroine found herself pregnant, unmarried, and alone. Sophie agreed to take in Christabel's illegitimate child and raise her as her own. She had her sister's back when it counted.
We have to assume that Sir George, Sr. consented to the arrangement, otherwise it would never have happened. Of the few other things that we know about Sir George, Sr., the most important is that he planted hundreds of the trees that now surround Seal Court—the very same trees that present-day Sir George protects "[l]ike some old goblin" (5.106). As our trusty narrator tells us, Sir George, Sr.:
[…] had had a passion for trees, trees from all parts of the distant earth, Persian plum, Turkey oak, Himalayan pine, Caucasian walnut and the Judas tree. He had had his generation's expansive sense of time—he had inherited hundred-year-old oaks and beeches and had planted spreads of woodland, rides and coppices he would never see. (8.42)
Talk about having a long-term vision.