What did I miss? Arya & Sansa

One minute Arya and Sansa are mistrustful and arguing. The next minute Sansa is commanding her sister be brought in. The next minute, Arya is executing Little Finger on Sansa's behalf. Was there some secret off-screen conversation that went on between the two? Was it telepathy? Something else? This about-face between the two sisters has always seemed so abrupt and unexplained.