Describe one of the novel's central dream symbols and what it means?I think it may be the one with Winston's mother and sister, but I'm not so sure...

It very much has to do with Winston's mother and sister, but not directly so.  Instead, the image of his mother and sister in the saloon of a sinking ship represents a death of the past and the family structure, which has been destroyed by Big Brother and the government's anti-family policies.  I would argue that another important link to the past as well as to nature is Winston's vision of "Golden Country."  This represents a return to nature and is in direct contrast to the gray, smoky surroundings that we encounter when in the city.  The city has been corrupted and distorted by Big Brother, but this return to nature in Winston's head is an escape to times past prior to the regime of Big Brother.