When you post, it is important to identify some criteria information about the persons you seek, their date of birth/place of birth are always helpful to volunteers. The year of birth or even an age of Marion at some known time (US Census, etc) would help find him. You did later identify that Sylvanus Collett was born about 1835 (where?).
--Based on the below research, it appears Marion was the son of Lydia Karren. Family tree records show Sylvanus married Phoebe in 1864. Phoebe was living with a new husband in the 1880 Census. Some family tree records also show Marion as being the son of Phoebe.
Without knowing what information you already have, your resources (member of Ancestry?) have your own posted family tree and other source material, I probably did more below than I should, as you may already have this data. There is a great amount of information online...and inconsistent without source documentation.
The 1856 Census of Lehi City, Utah County, UT shows a Sylvanus Collett. there were a number of other Collett in Lehi City, to include a Reuben (see below).
The 1860 Census of (unstated area), Cache Co, Utah Territory.
Syvaney Collett, age 25 (1864/1865) in England
Lydia, 21, England
Esther A, 1, Utah Territory
The 1870 Census of Logan, Cache Co, UT
Sylonus (as written/transcribed), 36 (1834/1835), Ohio
Lydia, 32, OH
--The census was enumerated on 22 July 1870, so Marion was born about April May 1870. So, it appears he was the son of Lydia.
--The 1880 Census of Smithfield, Cache Co, UT
William A Thompson, 47
Phoebe L, age 47
Samuel Collett, 13, Step-son
Mary Collett, 10, Step-dau
Marion Collett, 10, Step-son
---Marion was now living with Phoebe and a new husband. Below shows a family tree identifying Sylvanus did not marry Phoebe until 1864.
The 1880 Census of Cokeville, Uinta County, Wyoming
Sylvanus Collett, age 45 (1834/1835), born in England, a famer.
Nellie, wife, age 27 (1852/1853), born in Utah
Sylvester, 16 (1863/1864), Utah
Thomas, 14 (1865/1866), Utah
Reuben, 10 (1869/1870), Utah
Fanny, 8 (1871/1872), Utah
Nellie, 7 (1872/1873), Idaho
Robert W, 4 (1875/1876) Wyoming
Rose, 1 (1878/1879), Wyoming.
--This wife, Nellie, would have been about 10-12 years of age when Sylvester and Thomas were born...probably not their mother. So it appears their mother was Lydia. This Reuben was 10, the same age Marion would have been.
--So it appears that Sylvanus remarried between 1870 and 1880. There is no proof of the disappearance of Lydia (death, divorce, runaway, etc). The last known appearance of her was two months after the birth of Marion and Mary. Could this have been a difficult birth causing her later death? Could the "illiteracy" that you identified have been caused by a difficult birth versus lack of education. Many are called illiterate but the cause is often medical than educational environment.
1880 Census of Escalante, Iron County, UT
Reuben Collett, age 39 (1840/1841) born in England
had a son Sylvanus, age 13(1866/1867), born in UT.
--This represents a possible brother and nephew of the above Sylvanus.
The 1900 Census of Cokeville, Uinta Co, WY
Sylvanus, born May 1835 in England
Nellie, born Feb 1853 (shown as in England)
Burt, Aug 1880
Roy, Nov 1881
William R, June 1877
The 1900 Census of Browns Point, Routt Co, Colorado
Sylvanus Collett, born Dec 1866 in Utah, father born in England/mother PA
Sarah, born Oct 1865 in UT, three children all born in UT.
--The Western States Marriage Index shows a Sylvanus Collett marriage to Sarah E. Simkins in Cache County, UT in 1889.
The 1910 Census of North Ashley, Uintah Co, UT
Sylvanus, age 45 (1865) in UT, second marriage to:
Ethelwyn, age 35, married 7 years.
--The Western States Marriage Index shows marriage to Ethelwynn Stringham in 1904 in Salt Lake City.
I find a number of family trees concerning Sylvanus. While unsourced "facts" seem to spread on them, they show: He was born 3 May 1835 in Wellington, England, died 10 April 1901 in Cokerville, Lincoln County, WY. He married Lydia Karren in 1853. He married Phoebe Lodema Merrill in 1864. He died 10 April 1901 in Cokerville. Lydia is shown born 29 Dec 1838 in Liverpool, England, death on 15 Nov 1865 in Logan, Cache Co, UT. So there is a date problem between her death date and showing up in the 1870 Census and his marriage to Phoebe Merrill; unless they had separated, or being in Utah, this was a marriage of bigamy. Sylvanus father was Daniel Collett born in England, died 1894 in Smithfield, Cache Co, UT. A family tree found concerning Reuben Collett shows his father was also Daniel Collett. This "tree" shows Sylvanus having five spouses: Lydia Karren, Elizabeth Frances Preator; Sarah Jane Lawarence (born 1835 in Ohio); Lucy Ann Merrill, born 1808??? in Byron, New York; Phoebe Lodema Merrill, born 1832 in Elba, New York.
--In a second posting to this one, it appears his father was also married five times.
From the book "History of Utah: 1540-1886" pages 562-563:
Among other atrocities laid to the charge of the Mormons was one known as the Aiken massacre, which also occurred during the year 1857. Two brothers of that name, with four others, returnin from California to the eastern states, were arrested in southern Utah as spies, and, as was alleged, four of the party were escorted to Nephi, where it was arranged that Porter Rockwell and Sylvanus Collett should assassinate them. While encamped on the Sevier River they were attacked by night, two of them being killed and two wounded, the latter escaping to Nephi, whence they started for Salt Lkae City, but were murdered on their way at Willow Springs. Although the guilty parties were well known, it was not until many years later that one of them, named Collett, was arrested and in October 1878 was tried and acquitted at Provo. All the efforts of Judge Cradlebaugh availed nothing, and soon afterward he dischard the prisoners and adjourned his court sine die, entering on his docket the following minute: "The whole community presents a united and organized opposition to the proper administration of justice."
--There are also two postscripts at the bottom of page 563:http://books.google.com/books?id=1TAPAAAAYAAJ&pg=PA562&a...
In the book, Progressive Men of the State of Wyoming:http://books.google.com/books?id=gOMDAAAAYAAJ&pg=PA200&a...
is found quite a bio on Sylvanus.