Technically, Winston does not recall the deaths of his mother and sister; instead, he recalls the last time he sees them alive.
At the beginning of Chapter 7, Winston and Julia awake in the room above Mr. Charrington's shop. Winston is crying over a dream he had about his mother; he has believed for some time that he murdered her. He recalls now the times spent with his mother and sister hiding in underground shelters, often with little or no food. One day, out of desperation, Winston steals some chocolate from his family and runs away, never to see them again. This fact could have led to his belief that he inadvertently murdered his mother, since food was so scarce and his memories of hunger so vivid. Since we never meet any of Winston's family members, this conversation with Julia is invaluable to our understanding of Winston's background.