It would be and maybe Arya has just planned this all along, without telling sansa. She's just acting outraged to get to know the motives of Petyr and then decide to kill him. Another option is that Petyr is actually dead and is played by a faceless man trying to get sansa to kill Arya... I guess we will have to wait to know the truth
RE: Game of Thrones S7E06 Explained