Hello Michelle,
Here we are several years later and I am also investigating this part of the family. I've traced the Samuel line through the von Unwerths to the O'Dells, as well as William K and Donald B Calhoun's lines and have made contact with Donald B's grandson.
I am skeptical about Arthur and Samuel being brothers and think that Arthur may be a cousin or uncle to Samuel. I base this on being able to find 3rd and 4th cousins to Samuel, but no one who is related to Arthur...yet. Perhaps the letter from George Nethercut lends credence to him being a sibling, since he knew them.
I am descended from Samuel's brother William b. 1817, d 1883. The family was in Curraghamulkin (between Castlederg and Omagh and close to Drumquin) in Griffiths Valuation (I had Ulster Ancestry confirm this) in 1852. They had a farm, but William became a stone mason. The parents names were William Colquhoun (spelled the Scottish way in Griffiths, so he may have been born in Scotland) and Elizabeth Johnston (an Arthur Johnston was a witness to the marriage oftheir son William and Isabella Little, my 2x GGrandparents). I suspect that Elizabeth Johnstons' father's name may have been John, but have not yet found proof.
The brothers also had a sister, Elizabeth b. 1832. She married a man named John Cather, a shoemaker from Sixmilecross and they moved to Greenock, Scotland.
William and Isabella had 7 children: William John, Eliza Jane, Susan, Isabella and Sarah all born in Ireland, then my GGrandfather, James and Joseph, born in Glasgow. They moved to Scotland ostensibly after the parents died. Eliza Jane moved back to Ireland and married John Emery. I have been in touch with their descendants. James and Joseph married 2 sisters named Annabella and Rebecca Henderson, from the Castlederg area. James went to Glasgow and had 10 children and Joseph eventually set up a foundry in Douglas Bridge, Ireland and later a hardware store in Coleraine. I have been in touch with his GGrandchildren. Isabella married a man named William Futt Beggs and they came to the U.S. and have descendants in Massachusettes and Kansas, I believe. As a small child in Rutherglen, Scotland, my father knew Sarah, but said she was ill and stayed in bed most of the time. I don't know about Susan. I've been in touch with one of WIlliam John's descendants, who lives in Yorkshire, England after growing up in S. Africa.
I don't know if Laura is still out there, but it is exciting to find out more about the Arthur Calhoun line. I hope to touch base with our cousins here in the U.S.