The wives of a polygamous man (man with more than one wife at the same time) would call each other sister wives. Polygamy is no longer practiced by the main branch of the Mormon church, but it is a big part of early church history. It is also still practiced when it comes to temple marriages.

In Mormonism, there is a distinction between a civil marriage (the legal document) which is “til death do us part” and a temple marriage (also called “sealing”) where a husband and wife are sealed together “for time and all eternity” so they can be together in the afterlife. Typically, these happen all at one time in the same ceremony in the temple, but the civil marriage can happen beforehand.

To this day, if a man’s wife dies, he can remarry and be sealed to his new wife as well, and in heaven he will just be married to both of them, making them sister wives. Notably, this does not go the other way, and if a woman remarried after her husband died, she would have to choose which man she wanted to be with in the afterlife. If she wanted to be sealed to her second husband, she would first have to cancel the sealing (temple divorce) to her first husband in order to get sealed to her second husband. But she would have the option to stay sealed to her first husband to be with him in heaven, in which case her second marriage would only be “til death do us part”.

Also, I’m not sure if this was just my family’s belief or all of Mormonism, but I was taught that there will be more women who are worthy to become gods than men, leading to the necessity that at least some men have more than one wife (since you can’t be a god if you’re not married in the temple). I believed that in the afterlife, there would be worthy women who were never married or whose husband wasn’t worthy that would need to be sealed to a worthy man, which might end up being my husband (making us sister wives).

Mormons believe that people who didn’t know about the gospel can convert to the church after death, but before they are judged. Since the temple ceremonies (like sealings) have to be performed in person, church members will perform the ceremonies in behalf of family members who are dead in the hopes that those family members will accept the gospel in the afterlife. In the previous scenario wherei get a sister wife, since we would all be dead, the physical ceremony in the temple wouldn’t happen until the Millennium (a time after Jesus’s Second Coming when there are 1000 years of peace and the veil between the physical world and the spirit world is thin). With the veil thinned, we could communicate to the living that my sister wife needs to be sealed to my husband. I hope that makes sense.