Emma Smith was quite aware of the adulterous affair that her faithless hubby Joseph Smith had with one of his "adopted daughters," Fanny [Fannie] Alger:
"Benjamin Johnson, a close friend of Joseph Smith, described Fanny as, 'very nice and comely, [to whom] everyone seemed partial for the amiability of her character.”
"She is generally considered the first plural wife of Joseph Smith. Although undocumented, the marriage of Fanny and Joseph most likely took place in Kirtland, Ohio, sometime in 1833. She would have been sixteen years old.
"At the time, Fanny was living in the Smith home, perhaps helping Emma with house work and the children.
"Ann Eliza Webb recalls:
"'Mrs. Smith had an adopted daughter, a very pretty, pleasing young girl, about seventeen years old. She was extremely fond of her; no mother could be more devoted, and their affection for each other was a constant object of remark, so absorbing and genuine did it seem.'
"Joseph kept his marriage to Fanny out of the view of the public, and his wife Emma.
"Chauncey Webb recounts Emma’s later discovery of the relationship:
“'Emma was furious, and drove the girl, who was unable to conceal the consequences of her celestial relation with the prophet, out of her house.'
"Ann Eliza again recalls:
“' . . . [I]t was felt that [Emma] certainly must have had some very good reason for her action. By degrees it became whispered about that Joseph’s love for his adopted daughter was by no means a paternal affection, and his wife, discovering the fact, at once took measures to place the girl beyond his reach . . . . Since Emma refused decidedly to allow her to remain in her house . . . my mother offered to take her until she could be sent to her relatives . . . .'
"Book of Mormon witness, Oliver Cowdery, felt the relationship was something other than a marriage. He referred to it as '[a] dirty, nasty, filthy affair . . . '
"To calm rumors regarding Fanny’s relationship with Joseph, the [Mormon] church quickly adopted a 'Chapter of Rules for Marriage among the Saints,' which declared, 'Inasmuch as this Church of Christ has been reproached with . . . polygamy; we declare that we believe, that one man should have one wife . . . .” This 'Article on Marriage' was canonized and published in the 'Doctrine & Covenants.' In 1852, the doctrine of polygamy was publicly announced, thus ending eighteen years of secret practice. 'The Article on Marriage' became obsolete and was later removed."
("The Wives of Joseph Smith: Fanny Alger," at: http://www.wivesofjosephsmith.org/02-FannyAlger.htm
Mormon historians Linda King Newell and Valeen Tippetts Avery shed further light on the Joseph Smith/Fanny Alger affair:
"Emma [Smith] took nineteen-year-old Fanny Alger into her home early in 1835. Fanny's parents and brother were members of the church. Benjamin F. Johnson said . . . 'that Joseph LOVED HER.'
"But Joseph loved her indiscreetly, for Warren Parrish told Benjamin Johnson '[t]hat he himself & Oliver Cowdery did know that Joseph had Fanny Alger as a wife, for they were SPIED UPON & found together.'
"William McLellin told his account of Joseph and Fanny Alger to a newspaper reporter in 1875: '[McLellin] . . . informed me of the spot where the first well-authenticated case of polygamy took place, in which Joseph Smith was "sealed" to the hired girl. The "sealing" took place in a barn on the hay mow, and was witnessed by Mrs. Smith through a crack in the door! . . . Long afterward when he visited Mrs. Emma Smith . . . she then and there declared on her honor that it was a fact--"saw it with my own eyes."'
"In an 1872 letter, McClellin gave other details of the story. He said that Emma missed both Fanny and Joseph one night and went to look for them. She 'saw him and Fanny in the barn together alone. She looked through the crack and saw the transaction!! She told me this story too was verily true.'
"Joseph's theology may have allowed him to marry Fanny, but Emma was not ready to share her marriage with another woman. When Fanny's pregnancy became obvious, Emma forced her to leave. . . .
"The incident drove a serious wedge between Oliver Cowdery and Joseph. Oliver wrote to his brother Warren from Missouri on January 21, 1838: '. . . [W]e had some conversation in which . . . [a] dirty, nasty, filthy affair of his and Fanny Alger's was talked over in which I strictly declared that I had never deviated from the truth in the matter . . . . [J]ust before leaving, he wanted to drop every past thing, in which had been a difficulty or difference . . . .'"
(Linda King Newell and Valleen Tippetts Avery, "Mormon Enigma: Emma Hale Smith--Prophet's Wife, 'Elect Lady,' Polygamy's Foe" (Garden City, New York: Doubleday & Company, Inc., 1984), p. 66, original emphasis)
Historian Fawn Brodie (who placed the age of the orphaned Fanny at 17 when Joseph "seduced" her after she came to live with Joseph and Emma), described the affair as a "breath of scandal hot upon his neck," regardless of "[w]hether or not [she] bore Joseph a child." (Brodie reports, nonetheless, that "[t]here is some evidence that Fannie Alger bore Joseph a child in Kirtland"). Adding intrigue to the tryst, Brodie writes that "[w]hen in later years, polygamy had become an accepted pattern in Mormon life, Joseph's leading elders looked back to the Kirtland days and concluded that Fannie Alger had been the prophet's first wife. But when they questioned her about her relation with Joseph, she replied: 'That is all a matter of my own, and I have nothing to communicate."
Joseph's affair with Fanny was something that Emma could not easily forget. Indeed, Brodie observes that this "unfortunate infatuation" on Joseph's part for a "winsome servant girl" whom Emma had "taken into the family," absolutely incensed her:
"The scandal was insufferable to Emma, who was passionately fond and jealous of her husband. She had, moreover, a keen sense of the propriety and dignity of his office and must have been humiliated for the Church itself, which was beginning to attain stature and some degree of stability."
Brodie suggests that the affair ended up having a corrosive effect on Joseph's personal relationship with Emma, as hinted at "in November 1835 [when] he made a public statement [published in the 'Latter-Day Saint Messenger and Advocate'], part of which by its strange emphasis would seem to indicate that his domestic life was far from tranquil: 'Wives, submit yourselves unto your own husbands as unto the Lord, for the husband is the head of the wife, even as Christ is head of the Church. . . . Wives, submit yourselves unto your own husbands, as it is fit in the Lord.'"
(Fawn Brodie, ""No Man Knows My History: The Life of Joseph Smith, the Mormon Prophet," 2nd ed., revised and enlarged (New York: Alfred A. Knopf, 1983], pp. 181-83, 345)
Sounds like good ol' Emma had a lot to be mad at about--and a lot to hide. And that could have included knowing that her dear husband Joseph was (truth be told) a lying, conniving, untrustworthy snake, donchya think? But, hey, that just means, as Emma said, that he "was but a man except when the spirit of God was upon him."