In 1830, their mother was still Administrator for the affairs of Harriet and Mary which likely means they were still less than 18 years old.
From the LEXINGTON UNION, Lexington, MS. Jan. 19, 1840. Married in Attala county on Tuesday evening last by Rev. John Black of this place, Mr. James Devlin of Yazoo City to Miss Mary Haden of the former place.
Mary M. Devlin Will written 25 Feb 1841. Possessions left to mother. No children. Executor: brother, Jefferson. Witnessed by William Haden, Jr. "Logan County KY Wills 1838-1856" p.12; from Book G, p.192