History of Parliament for her son Henry has her as daughter of "Edward Ernle of Etchilhampton, Wilts." It has possibly the same father "Edward Ernle of Etchilhampton (who d.v.p., s. and h. ap. of Sir Walter Ernle, 1st Bt., M.P.)" for the third baronet.
Her son Henry is said to have married his cousin, which would fit with her being the 3rd baronet's sister.