I'm guessing it's going to be a parent of one of the adult couple, a sibling of one of the adult couple, or a friend of the married couple. It's possible that someone lived with them, or that one spouse felt safer with a friend or relative staying over, or needed help with the children, if the other spouse had moved out.
I guess another possibility is that it's a new partner of one of the adult couple, but until we get more information that suggests that the couple had been separated for a while or there had been infidelity, I'm less inclined to go that way. It's not impossible, but if - for example - the husband had been kicked out and the wife had moved her new partner in, in a heavily Mormon community that small? It would be all anybody would have been talking about. All the reporters would have been dished that tea already (except not tea, because Mormons don't drink that, so, pick your noncaffeinated bevvy of choice). It's possible if that is the case, it's all over social media, but I don't do that. I just hang out here. It's nicer. *blows kisses to the mods*