Where was Helen living (with her parents) in the 1940 census? (I found them in Fort Dodge in the 1930).
What was the date of the brother's obituary in which Helen's surname was given as Hill? Did that obituary give any indication where Helen lived at that time?
Do you have any additional documented location information about Helen between 1940 and 1999?
Are you certain that Helen died in Webster County, or was that simply the last known address on file with the Social Security Administration?
Have you called the Fort Dodge public library and asked them to check area city and telephone directories for Helen's name?
I found an obit online for a young man in Eagle Grove, Iowa; his late grandmother's name was Helen Hill, and his surviving grandfather was Elliott Hill. That might be worth pursuing as a possible match.http://www.legacy.com/obituaries/name/matthew-hill-obituary?...