You are missing the point of what was accomplished. the Starks presented a united front. As an all star team they have the king in the north, risen from the dead, who is also heir to the Iron throne. A superstar assassin. An all knowing and all seeing magical seer. And a hot redhead who is becoming a competent administrator.
The message with Bran there, and Arya carrying out the sentence wordlessly, is that they are one. It's team leadership, not Sansa in charge. They simply used the right tool for the job.