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I'm not sure if this is the Agnes Youngberg you are looking for, but my great-grandfather, Reuben, had a sister named Agnes, the daughter of John and Inga Maria Youngberg (originally Larsson), who were married in Vista, MN in 1896. Alfrid, Hulda, Carl, Lena, Theodore, Reubena, and Alfreda were their other siblings that lived to maturity.
Hope this helps!