The following demonstrate that Sarah Scott Mulholland Smith Mullinder was a plural wife of Joseph Smith Jr. Adapted From https://studylib.net/doc/7485962/

George D. Smith, Nauvoo Polygamy: "... but we called it celestial marriage", Salt Lake City: Signature Books, 2008, 218.
Four years later [1843], Sarah is thought to have married Joseph Smith in a union concealed by her October 25, 1843, civil marriage to Alexander Mullinder –a ceremony performed by Apostle John Taylor. On February 3, 1846, Sarah was sealed to Mulholland, Heber C. Kimball acting as proxy for her deceased first husband. There are two reasons for concluding that Sarah was joined to Joseph for time and eternity. First, in her 1846 sealing ceremony, she was identified as Sarah Smith, indicating a prior marriage to Joseph. [NO FOOTNOTE OR REFERENCE.] Second, in a biography of Heber Kimball, family tradition lists her among "the wives of the prophet" whom Kimball adopted as his own, 1844-46

Gary J. Bergera, "Identifying the Earliest Mormon Polygamists, 1841-1844," Dialogue: A Journal of Mormon Thought 38, no. 3 (Fall 2005): 31
Sarah Scott, who had married James Mulholland (1810-39) in early 1839, wed Alexander Mullinder/Mullander (born ca. 1810) civilly on October 25, 1843, with Apostle John Taylor performing the ceremony. Mulholland was probably a "front" husband to conceal Sarah's plural marriage to Smith—much the same arrangement by which Smith had authorized Joseph Kingsbury and Sarah Whitney's "prete[n]ded marriage" on April 29, 1843. Scott was sealed to Mulholland for eternity and to Heber Kimball, not Mullinder, for time on February 3, 1846, in the Nauvoo Temple. The record of that ceremony identifies her explicitly as "Sarah Smith," imply- ing an earlier sealing to Joseph Smith. Finally, Orson F. Whitney, son of Helen Mar Kimball Smith Whitney, Joseph Smith's youngest plural wife, wrote in his biography of his grandfather, Heber C. Kimball, that Mary Houston and Sarah Scott were known plural wives of Joseph Smith during Smith's lifetime [Whitney, Life of Heber C. Kimball (1888), 431; (1945), 41].

Todd Compton, In Sacred Loneliness: The Plural Wives of Joseph Smith. Salt Lake City: Signature Books, 1997, 631
Mary Houston (Kimball): "The wives of the Prophet who wedded Heber C. Kimball were ... Mary Houston." BOP #159; SAd 513. [BOP is "A Book of Proxey," Nauvoo temple proxy sealings, Jan. 7 to Feb. 5, 1846. GS and Marriot Library. SAd is "Sealing and Adoption Book A." GS and Marriott Library.]

Todd Compton, "A Trajectory of Plurality: An Overview of Joseph Smith's Thirty-Three Plural Wives," Dialogue, 29 (Summer 1996) 2:37.
SARAH SCOTT: Whitney, Life of Heber C. Kimball, 431. "The wives of the Prophet who wedded Heber C. Kimball were ... Sarah Scott." Cf. Sealing and Adoption Book A, 411.

Lisle Brown, Nauvoo Sealings, Adoptions, and Anointings: a Comprehensive Register of Persons Receiving LDS Temple Ordinances, 1841-1846, Salt Lake City: Signature Books, 2006, 215, 286.Scott, Sarah fn329: Name not in NTER [Nauvoo Temple Endowment Register]. Civil marriage to James Mulholland, 8 Feb 1838; Mulholland died 3 Nov 1849. [Plural marriage sealing for time to Joseph Smith, 1842-1843. Civil marriage to Alexander Mullinder, 25 Oct 1843 (Cook). For [plural] marriage sealing for time, see: Kimball, Heber Chase, and Mulholland, Sarah Scott (for time)...

[Mulholland, Sarah Scott (for time) fn 335: Name does not appear in NTER. Civil marriage to James Mulholland, 8 Feb 1838; he died 3 Nov 1839 (Bergera, Cook). [Plural] marriage for time to Joseph Smith, Jr., abt. 1842/1843, officiator's name not available (Bergera, Cook). For [proxy] marriage sealing, see Mulholland, James(d) and Scott, Sarah; and for [plural] marriage sealing for time,see :Kimball, Heber Chase, and Mulholland, Sarah Scott (for time). Civil marriage to Alexander Mullinder, 25 Oct 1845.

Lyndon W. Cook, Nauvoo Marriages Proxy Sealings 1843-1846, Provo, Utah: Grandin Book Company, 2004, 15, 190-91.
[6]James Mulholland (deceased) born 1804 Ireland andSarah Scott, [fn2] born 25 October 1816, Belfast, Ireland, were sealed husband and wife for time and all eternity, Joseph Smith Jr acting proxy for the deceased, by (unknown officiator), about 1843 Nauvoo, Hancock, Illinois. [fn4]Joseph Smith Jr. and Sarah Scott were seald husband and wife for time by (unknown officiator) 1842-1843, Nauvoo, Hancock, Illinois. [fn5]

[fn2]: Sarah Scott. Parents: Jacob Scott and Sarah Warnock. B 25 Oct 1816/1817 Belfast, Ireland d 25 Dec 1878 Waupun, Fon du Lac, Wisconsin. Alternate birth place Armagh, Ireland. Civil marriage too James Mulholland 8 Feb 1838 Far West, Caldwell, MO. Sealed for time to Joseph Smith 1842 –1843 Nauvoo, Hancock, IL. Sealed for time and eternity to James Mulholland deceased (Heber Chase Kimball proxy) 3 Feb 1846 Nauvoo Temple. Civil marriage to Alexander Mullinder 25 Oct 1843 Nauvoo, Hancock, IL. Sealed for time to Heber Chase Kimball 3 Feb 1846 Nauvoo Temple (divorced). (See Cook, Nauvoo Deaths and Marriages108 for record of civil marriage and Saints' Herald15 Feb 1879 for death date and place, and cause of death. Her remains were conveyed to Lyons, Walworth, WI for interment adjacent to her deceased daughter.

[fn4]: The actual date of this proxy ceremony is unknown, and it is possible that the "time" sealing occurred quite separately from the proxy marriage sealing.

[fn5]: The actual date of the "time" sealing of Joseph Smith Jr and Sarah Scott Mulholland is unknown but the fact that it occurred is not in question as may be perceived from an examination of entry No. 214.

[214]James Mulholland (deceased)Sarah Scott Oct 25 –1817 Belfast, Ireland were sealed husband & wife for time & all eternity by (H.C. Kimball acting proxy for the deceased. [fn4] H. C. Kimball & Sarah Smith [fn5] were sealed husband & wife for time by Amasa Lyman in presence of N.K. Whitney & Isaac Morley & Morris Phelps at 15 m. past 8 --F.D. Richards Clerk [fn6]

[fn4]: The vicarious aspectof this ceremony is repetitious inasmuch as Sarah Scott was sealed for time and eternity to James Mulholland (then deceased) when she was sealed to Joseph Smith Jr 1842-1843 Nauvoo, Hancock, IL.

[fn5]: At Nauvoo, subsequent to James Mulholland (then deceased) when she was sealed for time to Joseph Smith Jr 1842-1843 Nauvoo, Hancock, IL.

[fn6]: See "A Book of Proxy," No. 160. The text of the original record is retained in this entry.
***

From https://ancestorsbeta.familysearch.org/en/KWV8-Q6V/sarah-scott-1816-1878:
The Life Summary of Sarah
When Sarah Scott was born on 25 October 1816, in County Armagh, Ireland, her father, Jacob Scott, was 34 and her mother, Sarah Warnock, was 37. She married James Mulholland on 8 February 1838, in Far West, Caldwell, Missouri, United States. They were the parents of at least 1 daughter. She lived in Burlington, Racine, Wisconsin, United States for about 10 years and Wisconsin, United States in 1870. She died on 25 December 1878, in Wanikamiu, Fond du Lac, Wisconsin, United States, at the age of 62, and was buried in Lyons, Walworth, Wisconsin, United States.

Spouse
------
James Mulholland
1804–1839
Male

Marriage
--------
8 February 1838
Far West, Caldwell, Missouri, United States

Children (1)
------------
Helena Mulholland
1840–1906
Female

Spouse
------
Alexander Mullinder
1807–1878
Male

Marriage
--------
25 October 1843
Nauvoo, Hancock, Illinois, United States

Children (6)
------------
Jeanett Mullander
1844–1860
Female

Samuel Alexander Mullinder
1846–1908
Male

Ann M. Mullander
1848–1926
Female

Ruvenia Mullander
1850–Deceased
Female

Robert Thomas Mullander
1854–1935
Male

Sarah Victoria Mullander
1858–1946
Female

Parents
-------
Jacob Scott
1782–1845
Male

Sarah Warnock
1779–1841
Female

Siblings (8)
------------
Ann Scott
1805–1891
Female

Jane Scott
1806–1878
Female

Mary Scott
1808–1900
Female

John Scott
1811–1876
Male

Isaac Frank Scott
1814–1890
Male

Sarah Scott
1816–1878
Female

Jacob Scott Jr.
1820–1842
Male

Robert Thomas Scott
1824–1873
Male
Contributor: Steve Ingham (48083586)