Elie Wiesel actually has two, not one, surviving family members from the Holocaust. Both of his older sisters. Hilda and Bea Wiesel survived the death camps, although they were separated from Elie during and after the war. The rest of their family perished.
The Wiesel family was sent to Auschwitz in 1944. Auschwitz was not a single concentration camp, but it was a complex made up of three main camps. The family was divided by gender; Elie and his father were sent to one death camp, while his sisters and mother were sent to another. Elie actually had three sisters at the time. The youngest child, Tzipora, did not survive. From 1944 until the liberation of the camps, Elie did not have any contact with his female family members. He was reunited with Hilda and Bea after the war at an orphanage in France.