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Do you have these records from findagrave.com It seems that John and Abigail died the same year in different states according to findagrave. It does list Abigail's siblings.
Dear Ms. Phillips,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located the Population Schedules for the 1810 Census, the Population Schedules for the 1820 Census, the Population Schedules for the 1830 Census, the Population Schedules for the 1840 Census, the Population Schedules for the 1850 Census, the Population Schedules for the 1860 Census, the Population Schedules for the 1870 Census, the Population Schedules for the 1880 Census, and the Population Schedules for the 1890 Census in the Records of the Bureau of the Census (Record Group 29) that may contain information about John and Abigail Forrest in Ohio. The Population Schedules for the 1860 Census shows Samuel N. Forrest, age 5, as adopted and living with Isaac and Mary Oakes and others in Iowa. For more information about the non-digitized schedules, please contact the National Archives at Washington, DC - Textual Reference (RDT1) via email at email@example.com.
Due to the continued impact of COVID-19, you may experience a delay in receiving an initial acknowledgment as well as a substantive response to your reference request from RDT1. We apologize for this inconvenience and appreciate your understanding and patience as we balance mission-critical work and the safety of our staff during the pandemic. Please check NARA’s web page about COVID-19 updates for the latest information.
You may wish to search Ancestry or FamilySearch for the U.S. Census. There may be a fee for using Ancestry. Instead, please check for access at your local library as many library systems subscribe to these sites, making them free for their patrons.
We suggest that you review NARA’s Resources for Genealogists, as well as the History Hub Blog titled Suggestions and Advice for Family History Researchers. Also, the FamilySearch Research wiki for United States Genealogy may be useful.
We also suggest that you contact the Belmont County Clerk of Court to obtain a copy of the marriage record for John and Abigail Forrest. You may wish to search FamilySearch wiki Ohio Vital Records and Vinton County, Ohio Genealogy for how to locate 19th Century birth and death records. The obligatory recording on a state level of births and deaths in Ohio began on December 20, 1908.
We hope this is helpful. Best of luck with your family research!