I’ve read of families within the FLDS where the mother allows her young daughter to marry the stepfather. This then means that both the mother and the daughter are married to the same man. You may not understand how this could happen. But if you are raised in an atmosphere where natural societal norms are ignored, this type of marital incest is not only possible, but is sanctioned by stating it is required by the tenets of the religion.
Ann Young(ex/wife of Brigham Young) wrote about one case where a man married a woman who had young daughters. Part of the marriage agreement was that the girls would marry him when they were older. The little girls would talk about how someday they would ‘marry Pa.”
Nothing new with the FLDS doing it.