Hi
@davidthebuilder and welcome to the big reality check on biblical marriage. You are in exactly the same place most men here have been in or are in. So, welcome aboard.
You claim to be a Bible believing Christian. Okay, so start with Scripture.
But I want you to know that the head of every man is Christ, the head of woman is man, and the head of Christ is God (1 Cor. 11:3). NO ONE has authority to step between your wife and you! End of story. Point that simple truth out to her and she will be in sin if she determines to violate scripture.
Do you read the Bible daily for your family? If not, start yesterday. Start in Genesis and read to the Revelation, rinse and repeat. Polygyny is an inescapable reality in the Bible and it is evident in the lives of most of the great men, and of Almighty God Himself. Read without commenting on polygyny, just read through the Bible. Your wife and family will be confronted with the truth about polygyny as often as you will be. SAY NOTHING, let the power and authority of Holy Scripture accomplish God's purpose in your life and the lives of your family members. Shalom