Joseph learned about plural marriage in 1831; Alger is listed as "one of the first plural wives sealed to" Joseph.

May 1887
Andrew Jenson

Andrew Jenson, "Plural Marriage," Historical Record vol. 6, no. 3-5 (May 1887): 219, 232-233

Historical Record
Fanny Alger, Joseph Smith, Jr., Joseph F. Smith, Joseph B. Noble, Andrew Jenson
Reading Public

"The great and glorious principle of plural marriage was first revealed to Joseph Smith in 1831, but being forbidden to make it public, or to teach it as a doctrine of the Gospel, at that time, he confided the facts to only a very few of his intimate associates. Among them were Oliver Cowdery and Lyman E. Johnson, the latter confiding the fact to his traveling companion, Elder Orson Pratt, in the year 1832. (See Orson Pratt's testimony.) And this great principle remained concealed in the bosom of the Prophet Joseph Smith and the few to whom he revealed it, until he was commanded, about 1842, to instruct the leading members of the Priesthood, and those who were most faithful and intelligent, and best prepared to receive it.

. . . .Elder Joseph B. Noble next addressed the conference. He stated that the Prophet Joseph told him that the doctrine of celestial marriage was revealed to him while he was engaged on the work of translation of the Scriptures, but when the communication was first made the Lord stated that the time for the practice of that principle had not arrived. . . .Elder Noble sealed his wife's sister to Joseph, that being the first plural marriage consummated. The Prophet gave the form of the ceremony, Elder Noble repeating the words after him.

. . . .Fanny Alger, one of the first plural wives sealed to the Prophet.

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