Did she have a social security number? If she doesn't appear in the death benefit index, check her death certificate to see if it's on there.
If she did have one, have you sent for a copy of her SS-5 card? That's the application that she would have filled out when getting her SS number. It would have asked for the names of her parents.
Here's a link:http://genealogy.about.com/od/online_records/a/ss5_request.h...
Hope this helps,