What do we learn of Winston's memory of his mother?

Winston dreams of his mother, who disappeared in one of the great purges of the 1950s, when Winston was ten or eleven years old. In his dream his mother is sitting below him with his baby sister in her arms, in some sort of underground room. In this memory, he feels sad because he knows that somehow she has given up her life and that of his sisters for him. He thinks of her death as a tragedy - impossible in current times, because she died loving him and giving herself up to a personal loyalty that current Party rule would never allow.