It required superior loyalty and devotion to God to testify against a close family member or friend and then to be the first to share in the execution of that one. Of course, some persons might consider this to be too severe. But was it?
What if the idolatrous relative or friend were allowed to continue living and to influence others for wrong? This would have led to very serious, yes, tragic, consequences. Prostitution, homosexuality, drunkenness and child sacrifice were among the abominable practices associated with idolatry. (1*Ki. 14:24; Jer. 19:3-5; Hos. 4:13, 14; Amos 2:8) Think how damaging to the moral fiber of the nation of Israel such degrading practices would be and what sorrow and injury they would cause. Hence, the death of the idolater would shield many from the untold suffering that his influence would surely have brought if he were permitted to continue.
Today, of course, Christians are not authorized to execute idolaters. Nevertheless, it is still true that yielding to the influence of a close family member or friend to disregard God’s law can only spell disaster.
Don't know if your Mormon or not, but surely those who are.....consider it not the best when they marry outside of there beliefs, standards and principles, no?