She probably passed away very young (between 2 and 12 years). She appears on the 1900 census (age 2) and not on the 1910 census.
Sources
1900 Census: "United States Census, 1900" citing Affiliate Publication Number: T623; Line: 91; FHL microfilm: 1240896; Record number: 19603; FamilySearch Record: M3D3-QTQ (accessed 9 October 2022) FamilySearch Image: S3HT-DTYQ-JJP Image number 00405 Helen Shea (2), single daughter, in household of Thomas J Shea (34) in St. Louis, Missouri, United States. Born in Missouri.
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DNA Connections
It may be possible to confirm family relationships with Helen by comparing test results with other carriers of her ancestors' mitochondrial DNA.
However, there are no known mtDNA test-takers in her direct maternal line.
It is likely that these autosomal DNA test-takers will share some percentage of DNA with Helen: