At the very beginning of the book, on page 31, Winston has a dream about his mother. In the dream, his mother and sister are sitting in some deep place, drowning. But of them are looking up to Winston. He has a feeling that his mother has sacrificed herself and her daughter of him, and that he could not save them. That the sacrifice was the only way of having Winston survive. Winston says that the death of his mother was tragic in a was that was no longer possible (page 32).
"Tragedy, he PERCEIVED, belonged to the ancient time, to a time when there was still privacy, love and friendship, and when the members of a family stood by one another without needing to know the reason. HIs mother's memory tore at his heart because she had died loving him, when he was too young and selfish to love her in return, and because somehow, he did not remember how, she had sacrificed herself to a conception of loyalty that was private and unalterable. Such things, he saw, could not happen today. Today, there were fear, hatred and pain, but no dignity of emotion, no deep or complex sorrows." (page 32)
This time, the one Winston was referring to, is a time we have today. But imagine if we didn't live in a time like that... that we would live in a society of hatred and pain and loneliness and doubts and suspicions. Winston does not understand how or why, but he knows that he has been selfish to his mother and sister. But he does understand this: he knows that his mother had always loved him, always done her best to raise him, that she was loyal to him and sacrificed herself for him. He probably felt terrible for not loving his mother, for not being nicer to her, for being so selfish.
Later in the book, Winston remembers what had happened. He had a dream again, and this time he discovered a memory. He remembered that his father had disappeared, and that times were tough. He remembered that they lived in a tiny, dark place. He remembered always being hungry. He would always ask for more food, why there wasn't more food. He would scream and shout, he would cry when he couldn't get more. His sister was sick and dying, but he would still steal some of her food right from her plate. His mother would give him the biggest portion, but he would always ask for more. "He knew that he was starving the other two, but he could not help it; he even felt that he had a right to do it." (page 169)
He also remembered that one day there was a chocolate ration issued. He wanted to have all the chocolate, so his mother split it between he and his sister (he got 3/4 of it). However, Winston grabbed the piece his sister was holding, and ran off. He ate all the chocolate, and feeling guilty, wandered off into the streets. When he came back, his mother and sister were gone.