She is easy to lead. Petyr likes that maybe best of all, better than the way her mouth gives way when he kisses her or how much she resembles her mother if younger - better - than Cat will ever be again. Sansa is lithe and beautiful and clever, and although her mind has all the potential of an unsharpened blade he is the whetstone, this time, and whatever inherent obedience she inherited from her mother she now owes to him.
He has promised her a crown. He has promised her her people back, rule of the North when the time is right, and he has every intention of delivering on that promise. He is not a trustworthy man, but there are many things he is not that he will be for her. Sometimes he lets himself think she'll realize that someday, that she'll blink and see straight into the depths of him with those wide blue eyes. The thought chills him and even he can't tell if the sensation is pleasure or horror.
Until then, she is young enough that she is happy to have someone to follow, glad for the steadying voice of someone older and wiser and more experienced than herself, and she is so very good at following. Petyr thinks he could lead her into the depths of depravity if he tried, if he cared to destroy something so beautiful instead of perfect it. But she is an exception to his usual rule, as Cat used to be his exception. Cat is no more, now; sometimes he tries saying her name, just for the shocking lack of response it elicits in him now. It's just another name, all the weight and power of the syllables transferred seamlessly into Sansa's, all the want he has carried all these years drawn out by Sansa's features now while the habitual pain lingers in the past. It almost doesn't hurt at all, anymore, a little less every day.
And now there's only Sansa who could hurt him, if she set out to do it. But Sansa loves him, Sansa trusts him, and if he can only persuade her to never trust anyone else he will finally, finally have everything he has ever wanted. And if he can't? Then there is a betrayal in his future, an attack from the one person he will never be able to defend himself from, and in his heart he knows he'd be the more proud of her if she does turn on him, because in the end relying on others is a weakness, even if it's her on him.
But he's broken that rule himself, already, and aren't children supposed to surpass the skills of their parents?