If your grandmother knows any relatives who can help you, then it'll be fine but otherwise she has to fill out the document to get "Koseki". If she can get parents "Koseki" or "Joseki", you will know your grandmother's siblings' names and BD.

If you know anybody who can read Japanese, you can get the request form online.

There are more details but I don't know what you want to do so I don't want explain more at this point.