Does Wiesel ever finds out what happened to his mom and his sister? If so, does he ever see them again? If not, what happened to them?

In section 3 of Night Elie arrives with his family at Birkenau, the processing center for new arrivals at Auschwitz. The whole purpose of processing is to determine who's fit for work and who's going to be killed. Those deemed by the Nazis to be weak and therefore unable to work are to be sent to the gas chambers to die, after which their corpses will be burned in the camp's notorious crematory. Elie and his father both lie about their ages as they know that inmates considered too old or too young are liable to be sent immediately to the gas chambers. However, they are subsequently selected for work, and although Elie is understandably glad to be with his father, he also becomes separated from his mother and younger sister. He never sees them again, and we must presume that they were both murdered at Auschwitz along with so many others.