Isabelle Aboulker composed Lettre à ma soeur tant aimée (Letter to My Beloved Sister) for the Nadia et Lili Boulanger International Voice-Piano Competition 2021. The text is based on a recently discovered letter from Nadia Boulanger to her sister Lili that contains emotions of a rare intensity. The letter was written on February 7, 1917, during a period when Lili, isolated by the war and sickness, was spending a difficult period at Arcachon to recover her health.