In the book, it doesn’t state what happened to his mother. The only information that’” was given was Winston recalling the last memory of his mother. That memory being him stealing the last piece of chocolate rations from his little sister and running off. Later when returning home, he found them both gone, supposedly taken by the Party.
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. He has a feeling that his mother has sacrificed herself and her daughter of him, and that he could not save them.
