Thank you for that information.I have reviewed the will of John Cook you sent me, from Will book C, pages 449/450.
John Cook, husband of Catherine, died Jan-Mar 1831 ... father of sons Samuel, John, Henry, Daniel, Alexander and William ... father of daughters Susan (deceased?, m. Brogumier?), Elizabeth, Catherine (m. Jacob Fritz) and Charlotte
John's sibling: Henry Cook (deceased, with children)
John's sibling: George Cook (deceased, with children)
John's sibling: Mary Cook (wife of Abraham Sigerfoos or Seegefuse in the will)
It is clear here we have 4 siblings, John, Henry, George and Mary.There are no parents mentioned, which is typical of wills, but I believe it is still a possibility that the George Cook on page 61 of the Frederick County, Maryland 1790 census could be their father.In order for a father, mother, 3 sons and 1 daughter to fit, we only need one of the sons to have already married and left home or more likely not been born yet.Also, not only is Mary's brother named George, but so is her son John George.