Watch Your Mouth - S1.E29

Kendall swears Stacey to complete silence, and considering they're one of the biggest blabbermouth groups, they find it hard, until Kora gives them an offer. If they exchange one of them to Kora, she'll refresh the other twin's memories, of not only Kendall's fate, but of the fact that they never had a twin sister, as she will no longer exsist