CERSEI/SANSA IS MY FEMSLASH OTP IT IS PERFECT AND SOUL-CRUSHING AND WONDERFULLLLLLL.
I think that if Cersei had played her cards differently, she would have had Sansa in the palm of her hand, absolutely. Sansa saw her as an ideal, of the good and gentle and beautiful southron woman that she herself dreamed of becoming. She hungered for Cersei's approval so desperately and was crushed when Cersei turned out to be so different from what she thought. Also, Cersei was the main female presence in Sansa's life during puberty. That fact, taken in combination with Sansa's admiration for her, surely makes Cersei an important influence in the development of Sansa's sexuality.
I think that Cersei sees Sansa as a reflection of what she once was (or, what she should have been)- the Hand of the King's beautiful daughter, betrothed to the Crown Prince. But then, after Ned's murder and Robb's declaration of war, Cersei actively participates in the tearing-apart of Sansa's dreams and illusions. And instead of crumbling and admitting defeat, Sansa survives and grows older and more beautiful and more savvy about life, which I think terrifies Cersei.
SO BASICALLY THEY HAVE A VERY COMPLEX AND LOADED RELATIONSHIP AND I LOVE IT ALLLLL.
(And OH, don't even worry. When I type posts on my iPad, it takes me twice as long, because the typos just come out of nowhere.)