Who is XXX's sister?
Rose Leveson-Gower, Countess Granville
Rose Leveson-Gower, Countess Granville, GCVO (née Lady Rose Constance Bowes-Lyon, 6 May 1890 -- 17 November 1967) was the third daughter of 14th Earl of Strathmore and Kinghorne by his wife, Nina Cecilia, and an elder sister of Queen Elizabeth The Queen Mother.
Queen Elizabeth The Queen Mother