You're wise to deal with clearing up this matter, which is likely to get stickier with each passing generation.
The legal rules affecting real estate are set out and
controlled by the state in which the house is located"”and counties and municipalities sometimes throw in a few twists of their own. So you will need to do a bit of legwork or phoning about to see exactly what is required in your area.
Your first step, however, would be to visit the local title company or public land records office. While you might not be able to locate a copy of the deed to the house, there should be one on file at that office. And that document will tell you a lot about what needs to be done next. In the easiest scenario, for example, your mother might have owned the home jointly with her parents"”in which case, it will be quite simple to update the deed, or clear the title to it.
But in every locale and in every scenario, the end result will be the same: You will need to get a document on file that acts as legal notice that the former owners have died and identifying the current owner. The office that hands you a copy of the house deed should also be able to tell you exactly what is required to clear the title. So before you fret or fill out or file any additional documents, find out the specific hoops you will be required to jump through by the local authorities.
In some locales, it would be sufficient for your mother to simply file a certified copy of her parents' death certificates. If those aren't within reach, you should be able to secure them from the county or state Vital Records Office. In others, she would also be required to sign and file a statement setting out the facts of her parents' deaths and explaining that she is now the current sole owner.
If the person you deal with at the title company or county land records office is less than helpful, you can ask to see some recent title filings that were similar to your situation; they are all part of the public record.
With a bit of perseverance, you should be able to handle this on your own. But if things get sticky or overly confusing, consider consulting an experienced estate planning lawyer for help.