Hello to Margaret and Alan,
I've had a chance now to look more closely at the "Nelson/Stuebe" family tree on Ancestry.com. I'm afraid that, as with many Ancestry trees, much of the information is unsourced and unreliable. Once an error get posted on Ancestry it usually proliferates. The "Nelson/Stuebe" entry for James Miller has only two what I would consider bona fide sources: James Miller's land purchase and his grave in Log Church Cemetery. The immigration record is, I believe, another man's, as the date doesn't seem right. As for other names in the "Nelson/Stuebe" tree, there was no second marriage for James Miller (b. 1798). He died in 1862 and his widow, Susan (Foyle) Miller (the mother of all his children) didn't die until 1878. When she passed, she was living among her daughters in Cottage Grove, Minnesota -- where she is buried (near my own parents and brother). The name "James Kirtley Miller" also seems to have snuck into Ancestry from another individual's records. Our first-generation family members didn't have middle names -- that came in with the second generation, I believe.
I can see where this genealogist was confused about "David Grey Miller," because he cites the 1850 census for Wappello, Jo Daviess County, IL. This is where one of the sons of Joseph & Martha (Humphreys) Miller lived. He had married a woman named Elizabeth Gray. She had a disabled brother, David, who lived with John and Elizabeth into adulthood. The story of their inclusion of him in the household and their care of him told in a county history book, "Portrait and Biographicl Album of Jo Daviess County, Illinois" (pp. 446-448), published by Chapman Bros. in 1889 and available online at https://archive.org/stream/cu31924028805402#page/n7/mode/2up This book has a lot of information on other Irish immigrants from Monaghan to this part of Illinois. In fact, the Jo Daviess county area where many of them lived was called "Irish Hollow." They started arriving there well before the Homestead Act no doubt drawn by their community of relatives and fellow Presbyterians from Monaghan.
Loads of people with the surname "Gray" or "Miller," so it is a complex research.
Alan, your hunch about George Miller is correct -- he was the older brother of James and Rosamond. I will send you more information on him via email.
Best regards, Gretchen