The answer you seek is contained in my book "The New England Ancestors of the Immigrant, Ralph Farnum of Rochester, Kent County, England and Ipswich, Massachusetts" (1999, Peter E Randall Publishers, Portsmouth, NH), pp 631-632. Dorothy's ancestral lines back to the immigrant, Ralph, and his father, Henry, are contained in my book. Go to http://www.farnhambook.com. Dorothy's descent is from my line, making us cousins.
Indeed, Dorothy was a "Farnham", who had two marriages. Your grandmother Betsy Ann, who m Wentworth Stevens, was never followed further, beyond her marriage.
A Volume 3 is in the hopper and I would have interest in any documented input you may wish to share.