In a context where LittleFinger is trying to split the Stark sisters and mentions Brienne, Sansa decides to send Brienne to King's Landing. I can't decide why she would do that :
- Either to protect herself in case she (Sansa) planned to harm Arya
- Or to protect Arya from the possibility that LF would manipulate Brienne to harm Arya
- Or maybe it's not related, Sansa sends Brienne because she's the only person she trusts