Maine
All of the people here were born in Maine, though they served in units from Maine, Massachusetts, and Michigan. All are McLellan relatives, by birth or marriage, and they fought for the North in many of the same battles in which their cousins from Louisiana fought for the South. None are closely divided families, but they do represent cousins fighting cousins. William H.P. McLellan only moved to New Orleans in 1840, and his family often traveled back during summers to avoid yellow fever; his family must have known of the cousins they were fighting against. Here is 
a guide to research of Civil War ancestors from Maine. The Maine State Archives have a page about the War.
 Dr. Henry Levensaler, 8th Maine Infantry |
According to Eaton, James Brackett McLellan (a brother of William H.P. McLellan) served in Company E of the Maine 1st Regiment of Heavy Artillery in 1863. I have not, however, been able to find his name on rosters, so this needs to be checked. He later moved to New Orleans himself.
Dr. Henry C. Levensaler served as a surgeon in the 19th and then the 8th Maine Infantry Regiments. His first cousin, also named Henry Levensaler, fought in Virginia for a Massachusetts regiment, and was killed in May of 1864 during the Wilderness campaign. Henry therefore fought in battles against Capt. Charles McLellan, the son of his first cousin Leonora Levensaler, who had moved to Louisiana some twenty years earlier. Dr. Levensaler was her Leonora's nephew. Henry died in Virginia just eight days before Charles, in the same campaign.
Brig. Gen. John Marshall Brown entered the war in 1863 at a captain, fought at Gettysburg, in Georgia and South Carolina, and then at the end of the Overland Campaign at the Anna River and Cold Harbor before being seriously wounded at Petersburg; he was breveted to Brigadier General by the end of the war. His wife, Alida Carroll, was the brother of Gen. Samuel Sprigg Carroll of Washington, D.C. (see below) and the sister-in-law of Gen. Charles Griffin of Ohio.
The Edwards family connect to the McLellans because two Edwards brothers (Samuel and Enoch) married two McLellan sisters (Martha and Abigail, respectively). Samuel's grandchildren Albert S. Estes and Dana Estes were both privates in the 13th Regiment, Company A of the Massachusetts Volunteer Infantry. Albert was killed at 2nd Bull Run on 30 August 1862; his brother Dana was wounded the same day.
Samuel's brother Enoch Edwards had two sons and four grandsons who served in Maine infantry regiments:
Two of his son Lathrop's children served. David Andrews Edwards entered the 5th Maine Infantry as a corporal and was later wounded at the "Bloody Angle" at the Battle of Spotsylvania, one of the grimmest fights of the War. Albert Marshall Edwards had moved to Michigan. He at first served in the 1st Michigan Infantry Regiment (Company K), and was captured at 1st Bull Run. After being paroled, he returned home and served again as a Captain in the 24th Michigan Infantry (Company F). He fought at Gettysburg, Mine Run (his cousin Capt. Charles McLellan was wounded there), Cold Harbor (where Charles McLellan was killed), Weldon Railroad, and ultimately mustered out as a Colonel.
One of the sons of his daughter Mary served. George Wilder Kimball served in the Maine 12th Infantry (Company A), and died in New Orleans on 13 November 1863. Among other engagments in Louisiana, the 12th fought at Port Hudson, where the Pitards and Gamards fought, in May-July of 1863.
His son Bryce McLellan Edwards served, and so did one of his sons, Sydney Danforth Edwards, both as privates. Bryce served in the 5th Maine Infantry (Company I), and apparently was discharged during the war as wounded. Sydney served in the same Company, and served in the Red River expedition in Louisiana. (You can read about this and other "Military Adventures Beyond the Mississippi" in an essay from Harper's kept on the Texas Histories page).
Enoch's son Clark Swett Edwards also served in the 5th Maine Infantry. He, however, mustered in as a Captain in Company I, and mustered out as Brigadier General. He, and the 5th Maine, fought at First and Second Bull Run, Fredericksburg, Chancellorsville, Gettysburg, The Wilderness, Spotsylvania, and Cold Harbor.
Massachusetts
Capt. Alpheus Hyatt served as a Capt. in Company A of the 47st Massachusetts Regiment, which served in the New Orleans area during the War.
Henry Levensaler ("Levensalor" in his war record) from Waldoboro, Maine was a private in the 22nd Massachusetts. He was mortally wounded on May 8th, 1864 at Laurel Hill, Virginia.