Do you know the name of Samuel and Sumner Burrill's mother?
Do you know when Samuel and Clara Burrill died and where they are buried?
Do you know anything about Julia A. Sewall, who was 22 in 1840 when Melville and Martin lived with her at Corinna? I did not find a gravestone inscription for her at Corinna.
Rozetta, Henderson Co., Ill., where Sumner's wife and two of their children were found in 1870, is on the Iowa border, and LaSalle Co., where Martin was found in 1860 and 1870, is about 115 miles NNW of Henderson Co. Sumner and Jane lived in Newport, Maine, in 1860 (Penobscot Co., 231). My first guess is that they moved to Rozetta together and he died there.
Is the letter mentioning Susan's burial dated 1867 or 1877? Susan was enumerated with Samuel in 1870. Is "SJ" Samuel? I have no middle names for either Suamuel nor Sumner.
Unless I have erred in my compilations, there is a chronology problem with saying that Sarah Jane Sewall was the sister of Martin's father. Sarah Jane Sewall did have a brother named William Dummer Sewall, but he was born in 1813 and died in 1862 or 1863.
I have no record of a marriage or children nor of what he did.
This William Sewall's will says he was the son of Dummer Sewall late or Corinna. His will mentions no wife or children -- he leaves everything to Nathan and Nancy Merrill, of Saint Albans. Martin was clearly alive in 1862, and one of his daughters was born in Maine in 1868.
There is another William D. Sewall, whose dates would be a perfect match: born in 1804, and is said to have died in Illinois in 1839, but he comes with a wife and three children born in Chesterville, 1828-33. Now if his wife died and he married Susan ca. 1833, he could be the father of Justin, Melville/Melvin, and Martin, who were born 1834-37. There is no evidence yet of such an alternative.
I am fascinated that you live on the Burrill place. The world is indeed flat.