I don't have any information on James McCulloch,the brother of Samuel McCulloch, but here is some background information on how McCulloch's Mills came into being and Samuel McCulloch's role in same.
On June 12, 1793, John Anderson (Commissioner 1851, Juniata County) conveyed the tract of property which was to become McCulloch's Mills to James Anderson(Commissioner 1856, Juniata County). James Anderson built the third grist-mill in the Tuscararo Valley and the first grist-mill upon the one hundred and ninety-three acres tract of land that was to become McCulloch's Mill with a completion date of May 13, 1786.James Anderson built a home on the property in 1800. James Anderson sold the home, grist-mill and tract of land to Samuel McCulloch on March 26, 1804. The cornerstone for the James Anderson home is still visible in the foundation of the existing Samuel McCulloch home. Samuel McCulloch, a native of Scotland, came to Pennsylvania with his four brothers, either from Scotland or Northern Ireland, locating in the Tuscarawas Valley. Records state that "Samuel McCulloch of McCulloch Mills died, age 88 years, January 2,1860" (see the Pennell Files). This would mean that Samuel McCulloch was born in 1772 and he was 21 years old when he established his whiskey distillery at McCullochs Mills. The 21-year old Samuel McCulloch began operation of his whiskey distillery in Lack Township (later Tuscarora Township) on November 13, 1793 (see the Pennell Files) and he was assessed as a merchant in Tuscarora Township in 1803("The exports of Juniata county are whiskey and wheat". See the Rev. Shindle's "Historical Sketch of Juniata County", dated July 20, 1876 from the Port Royal Times). Samuel McCulloch's whiskey distillery shows up on the Lack Township (later Tuscarora Township) tax records in 1797 and the distillery remained on the tax records as a going concern until 1831. As a whiskey distiller, Samuel McCulloch would have been a sizable customer for cracked corn and barley for making the sour mash from which he would distill his whiskey, so it would have been a good business move for him to keep his eyes open for a business opportunity that would have included the purchase of a grist-mill where he could crack the grain he purchased for making his whiskey. James Anderson sold his grist-mill, home and tract of land to Samuel McCulloch on March 26, 1804. Samuel McCulloch added a saw-mill to his operation five years later and it began operation in September 1809. Samuel McCulloch rebuilt the grist-mill in 1813. David Nugent was the millwright. After the purchase of the real property and improvements from James Anderson in 1804, the town became known as McCulloch's Mills. The McCulloch Mills Cemetery indicates that the town was started in 1800. Samuel McCulloch also transported milled grain from his grist-mill and other grist-mills in the area to Baltimore for sale. The milled grain was transported in a large and sturdy wagon pulled by a six-horse team run by employee, Jemmy Given. Jemmy Given was "McCulloch's wagoner, and so honest and faithful was he in freighting for McCulloch between the mills and Baltimore that he became a necessity to McCulloch and drew high wages. Though a small man, but very stout.."(see the funny March 7, 1889 article "Olden Times Wedding" in the Port Royal Times).
Samuel McCulloch married Priscilla Turbett, daughter of the Thomas Turbett, in whose honor Turbett township was named (year ?). Their children were: John; James; Thomas; Samuel; George; Stewart; Nancy; and Jane. Samuel McCulloch was a merchant in McCulloch's Mills for the remainder of his life. The county business records reflect that Samuel McCulloch closed his carding machine operations on March 4, 1825. He went into a business partnership with Joseph Laird for "the countenance of Mr. McCulloch's merchant business" in Tuscarora Township in February 1828. Samuel McCulloch served on the first Petit Jury on December 5, 1831 and he succeeded Joseph S. Laird as Postmaster of McCulloch's Mills on December 5, 1831. Samuel McCulloch and his wife died at McCulloch's Mill. Samuel McCulloch died January 2, 1860.