Joseph Smith was sealed/married to his (effective) foster daughter, Lucy Walker May 1, 1843. We know that JS introduced plural marriage to her brother William in the early 1840s, and he then asked William (23 yrs old) for permission to marry his younger sister (who was 17). Please note the verbiage William used to describe the situation:
We were riding all day through the city and county making a number of calls on business [and] pleasure combined. On this occasion the subject of celestial, or plural marriage, was introduced to me. As we returned home he remarked, “If there was anything I did not understand, to hold on a little, and I would understand it.” William Holmes Walker, Reminiscence and Diary, 7–10, CHL, Ms 1890. (see here)
Interestingly, William would shortly go on to marry Olive Hovey Farr on Nov 1, 1843 in Nauvoo, Illinois.
Family search records suggest that the couple were not “sealed” until 1850 (28 April, 1850 in SLC Utah). An absence in Bergera’s study of early couples who were sealed together further supports this later sealing date.
This episode further supports the idea that “sealing” was primarily about plural marriage. And this supports the idea that Joseph Smith was not merely being altruistic in his sealing to Lucy Walker. Apparently JS didn’t bother to ensure that William was sealed to his wife Olive in late 1843, even though William clearly had accepted the principle of “celestial marriage” and gave permission for his sister to be sealed to JS. Every record suggests that William was highly faithful (for instance, was part of the Mormon Battalion and led a wagon train to Utah), but apparently sealing to his wife was of no major concern for anyone.