Cersei Lannister tells Margaery Tyrell the history of House Reyne – Game of Thrones Season 3 Ep 8

At the High Septon hall, Margaery calls Cersei her "sister". Cersei tells Margaery the tragic history of House Reyne of Castamere, as reflected in the song "The Rains of Castamere". Cersei threatens Margaery that if she ever call her "sister" again, she'll kill her in sleep.