There were dozens of Harbisons in SW Indiana in 1840. But they were almost all migrants from Kentucky and South Carolina. And I can find no record of William there, other than the letter. What seems to have happened was he went north to Illinois, and on September 1, 1845, bought 40 acres in McHenry County, very near the Illinois-Wisconsin state line.It's the far top left quarter of the deeper orange box on the map. It's close to the modern intersection of Thayer Road and Illinois Hightway 27. It looks like typical midwestern farmland; but 40 acres (a quarter of a quarter section) is not much. Most of his neighbors, according to the 1850 census, were Irish.
Henry was according to the 1850 census, born in 1844 in Illinois, but his death record says Jan 10,1847. He worked as a coachman in Milwaukee, and was married in a Methodist Episcopal Church on Biddle St in Milwaukee on December 2, 1885 to Louise Hoffman, daughter of Peter Hoffmann and Katherine Schmidt, from Bavaria. On his death certificate he was listed as a toolmaker or machinist in a watch factory, and died on Mar 31, 1927 in Elgin, Kane Co., Illinois, where he had lived 32 years. His father is listed as being from Belfast, Northern Ireland; his mother from Dublin. He and Louise had no children.
William J Harbison's death certificate lists him as being born on April 12, 1844, but given the 1850 census record and the date, it's likely to have been 1849, and Mary Elizabeth was his twin sister. He mustered into Company K of the 153rd Illinois Infantry on Feb 11 1865, apparently guarding the Nashville and Chattanooga Railroad and out on Sep 21 1865. He is listed as divorced and living with his parents in 1880. Divorce would be unusual for a second-generation Catholic immigrant. There is no record of any children. I can't find him in 1900 or 1910; in 1920 he was living in the Illinois Soldiers and Sailors home in Riverside, Adams Co. Illinois.
Finally, Mary Elizabeth Harbison, b. April 12, 1849, married William Lindsey, from New York, on July 4 1867, and had five children I can trace: Mary (b. 1868); Ebenezer (b. 1869); Dora (b. 1873); Ira W (b. 1874) and Raymond (b. 1877). By 1900 she and her husband had moved to Elgin, Kane Co. Illinois, which is now a suburb of Chicago. Raymond, a printer, and Ira died in 1905 and 1917 respectively, causes unknown. I can find out very little about them, and less about Ebenezer and Mary, who disappear after the 1880 census. Dora married a man named Hubert H Loomer, from Wisonsin, and had a child Alfred. She still has a living descendent. Mary Elizabeth died 27 April 1933; her husband died in 1917.