The only two daughters who could possibly have died in the lightening strike are Lucy and Hannah, the obviously youngest daughters. The other daughters are accounted for and there were no other daughters. If Lucy had survived there would have been big news in the papers about her survival.

Will of Benjamin Harrison written 17 Oct 1743 Charles City County, Virginia
names wife Ann
son Henry dead and buried on Berkley Plantation (to be decently buried on Berkley Plantation near my little dear son Henry's grave)
son Benjamin
son Carter Henry
son Henry
son Robert
son Nathaniel
son Charles
daughter Betty - (see below obtained letters of administration, who married in 1746 Peyton Randolph, 1st President of Continental Congress, had no children)
daughter Anne (married William Randolph III of Wilton, see below obtained letters of administration)
daughter Lucy
daughter Hannah (see Maryland newspaper, 1745 struck by lightening)

Charles City County, August Court 1745
The above written Last will and Testament of Col Benjamin Harrison Dec'd was presented in court by WILLIAM RANDOLPH Gent and WIFE (NOTE: WOULD BE ANNE HARRISON) and MISS BETTY HARRISON, WHO MADE OATH there and being proved by the oath of Joseph Royall one of the witnesses thereto is admited to Record, and the said Joseph Royall also on his oath declared he saw John Stith and Robert West the other two witnesses sign the said will as witnesses, and ON THE MOTION of the said WILLIAM RANDOLPH and MISS BETTY HARRISON, and performing what is usual on such cases. CERTIFICATE is GRANTED THEM FOR OBTAINING LETTERS OF ADMINISTRATION with the said will annext in due forme Durante Minori estate of Benjamin Harrison son and heirs of the Dec'd.
Test Lewellin Eppes, Clk. Curia

THE WIFE OF WILLIAM RANDOLPH WAS ANNE HARRISON, DTR OF BENJAMIN. Both girls would had to be at least 18 years old to qualify to administer the estate. So Lucy and Hannah are the two youngest girls. Son Benjamin would have only been 19 years old at the time and not old enough to serve as executor.

Maryland Gazette, Annapolis, July 18, 1745
Last Friday evening July 12 a most terrible accident happened in Charles City County; when a violent thunder gust arose, and lightning struck the house of Col. Benjamin Harrison, of Berkley, which kill'd him, and his two youngest daughters. ... The most melancholy affair was given a general concern, and particularly to all who were acquainted with the deceased. Col. Harrison had been many years a worthy representative of his county in assembly, and first in the commission of the peace, and his daughters very promising young ladies.