There is a lot of meaning in that dress.

Not only is it black, signifying the death of ‘Sansa Stark’, but it’s sprouting black feathers in its shoulders, generally the Westerosi houses dress according to their colors, Cersei with the Reds and Golds of the Lannister, Olenna and Margaery with the blues and greens with the roses embroidered, the Baratheons in Gold and Black, the Greyjoys in dark grey and deep blue, and the Starks in white and grey.

Alayne is wearing the feathers of the Eagle, the animal of House Arryn, her dress symbolizes one thing, for all intents and puposes, once Robin is shipped off, she is the Lady of the Vale of Arryn and protector of the Vale, and what she did against Petyr’s wishes secured her safety, she knew as long as the Lords declarants of the Vale knew she was Sansa Stark she would be protected from Petyr and she would also be free to ploy with him at the same time playing as Alayne in disguise.

Now the Vale of Arryn is sworn to her, the cornerstone of her plan was that Lord Yohn of House Royce KNEW who she was as he visited Wintefell when she was a Little Girl, something only the REAL Sansa Stark would know, proving her name to them.

Sansa just proved that she’s learned to use her sad little innocent girl to her advantage and has been doing it this whole time.