It was definitely an unexpected heel turn and a bit of a departure and maybe a overshadowing of things to come from her storyline. I actually liked Sansa as a character in the book and I wasn't too bothered with her in the show. She slowly came to realize her position and power within the game and she sees the writing on the wall and by aligning herself to Petyr will put her in a powerful position. I especially liked how Sansa put Petyr in his place like I know you want this.

For all intents and purposes in her mind, her entire family is dead and she is the only survivor and she wants to live so she's finally taking that ownership over her life and is getting something out of it. The question will be will she actually bump into Arya, I doubt it but the thought did occur to me what if in the next episode they do reunite but they reject one another and a fight breaks out between The Hound and a Knight of the Vale and that's when he gets his fatal wound.