One of the biggest problems with witnessing to mormons is that we are using many of the same words and often even referencing the same Scriptures, but meaning entirely different things. This exposes the lack of Biblical depth that most Christians, including me, have because we have a hard time exposing the context and rebutting what is being said with the entire Biblical record. When we are witnessing to mormons and others who are lost, we need to understand what the Bible says so that when faced with verses out of context we can draw the conversation back to the Bible instead of arguing about what a verse means.
That is nowhere more true that the Biblical doctrine of adoption.
Mormons read in the Scripture that we will become joint heirs with Christ, that we will be called sons of God and immediately connect that to the mormon doctrine of exaltation (i.e. that the most faithful mormon men will become gods and reign over their own planets, just as the God of the Bible is supposed to be an exalted man. It is all a divine pyramid scheme in a way). This is an all too common reaction from mormons who typically view all of Scripture with suspicion, and whenever possible view any doctrine through the lens of mormonism. But it is clear that the Bible tells us that there is but one God, negating the idea of man's exaltation in the mormon sense.
So if the mormon idea of exaltation, of deification of man is not what the Bible is speaking of, what do these verses mean? In a nutshell, they refer to adoption. There are a number of passages that directly deal with adoption in the New Testament...
For all who are led by the Spirit of God are sons of God. For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, "Abba! Father!" The Spirit himself bears witness with our spirit that we are children of God, and if children, then heirs--heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him. (Romans 8:14-17 ESV)
But when the fullness of time had come, God sent forth his Son, born of woman, born under the law, to redeem those who were under the law, so that we might receive adoption as sons. And because you are sons, God has sent the Spirit of his Son into our hearts, crying, "Abba! Father!" So you are no longer a slave, but a son, and if a son, then an heir through God. (Galatians 4:4-7)
Blessed be the God and Father of our Lord Jesus Christ, who has blessed us in Christ with every spiritual blessing in the heavenly places, even as he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love he predestined us for adoption as sons through Jesus Christ, according to the purpose of his will, to the praise of his glorious grace, with which he has blessed us in the Beloved. (Ephesians 1:3-6)
So what we see in these verses that deal directly with the becoming sons of God, joint heirs with Christ, is that we are not granted this because we are by nature like Christ but because we have been granted that status graciously through adoption.
There is a great sermon by C.J. Mahaney on the topic of adoption that you can listen to here that really gets at the humbling heart of adoption. We are not made God's equals or even like Him in nature. Jesus is not our literal brother. What we receive though is the blessing of renewed familial fellowship with our God through the purchasing power of Christ's blood. Salvation is not merely being saved from hell, it is also being adopted by God, given the glorious benefits of being sons and daughters of God. If we were merely saved from hell, that alone would be worthy of an eternity of gratitude and praise to God, but God is not content merely to save us but also to adopt us, calling us out of the world of sin.
We were once slaves to this world, slaves to sin but when we come to faith in God through His grace, we are no longer slaves, but sons. If you adopt a child, that child is not genetically made like you. They are still different from you in a number of ways, but they are given the same rights as your natural children. God only has one Son, His only begotten who is like Him but we are ushered into His glorious presence through adoption, and that is why the Christian can call God "Father".