This site is from St. Louis, Missouri, but what it says should hold true anywhere:
http://www.slpl.lib.mo.us/libsrc/stl-death.htm
It says:"Before they elected to require statewide keeping of vital records,many states had a period when keeping of vital records (usually by county clerks) was discretionary. Compliance wasoften low, but some county clerks did keep vital records during this trial period. Sometimes copies of such records were sent to a state office; often, however, they were kept only by the county office. You should check with the COUNTY CLERK [sometimes called the CLERK OF COURT] in the county where a person died to see if there is a death certificate, even if there are supposedly no death records for that period for that county.
CORONER RECORDS: If an ancestor died an unnatural death, especially in a big city or well-established county, there is a chance that a city or county coroner held an inquest to determine cause of death. This may be so, even if that state at that time did not mandate keeping of vital records. Coroner case files are usually well-organized and indexed.
HOSPITAL RECORDS: If your ancestor died in a hospital, there is a chance a record of the event still exists. This is especially true if the ancestor died in a city hospital, quarantine hospital, or other government-run health-care facility. Such a record may exist even if the state did not require the keeping of vital records at that time. A hospital which did not maintain a special
register of deaths may have made note of patient fatalities in the hospital's register of admissions and discharges.
[It could be that WRITTEN records were thrown out, but they should be there, even if on microfilm/microfiche/or computerized. For instance, EVERY record of my own doctors' visits are on computer at my local hospital, and they date back 30+ years. It is also possible, when you asked about them, they figured "How do we know this girl is related to Mrs. X.?? She could be ANYBODY." In this case, if your mother is wanting to know as well, have HER call about them. She was a daughter of the woman, after all.
One final thought: for her to have died at such a young age, it could have been from a number of things--from a botched abortion to inherited diseases. If there IS some genetic diseases in your family, you and your mother should have the right to know about it.
Also try http://www.newspaperarchive.com/Default.aspx. You can search for her obituary there if you know the date and place she died. They cover the years 1700-2009!!]