I am curious about this particular Mormon family. We know mainline Mormonism has abandoned the practice of polygamy, but there are offshoots of Mormonism that have made inroads in converting mainline Mormons to their particular offshoot. I have read I and II Nephi and have gotten more than a few chuckles out of it, so I would take everything you hear from your interactions with Mormons, with a grain of salt, or perhaps a shaker full of salt. The Book of Mormon itself, condemns polygamy and refers to it as sin and an abomination. The Holy Bible OTOH, gives no such condemnation, and there is plenty of Scriptural evidence to show that polygamy is acceptable. Of course, we could go into Egyptian papyrus, and discuss how Joseph Smith supposedly translated that into the Book of Abraham. The Mormon church still holds to the authenticity of the Book of Abraham, even though it can be clearly shown that the Egyptian papyrus from which it was supposedly translated, has nothing to do with what it was supposedly translated into.
We at BF love Mormons, and agree with them on many of the family values that they stand for, but by and large, we disagree with them on Theological issues. It's sad, because those offshoots are well established, and if they had a proper understanding of the nature of God, Jesus, Satan, the authority of the Holy Scriptures as genuinely revealed by the Holy Spirit to the prophets and apostles, and if they understood the means of salvation correctly, we could have great fellowship and communion with them!