JOEL - I think you are talking about two different things here. One is talking about literally being the spirit sons and daughters of God the Father, the other is about figuratively becoming sons and daughters of Jesus Christ.
LDS believe that in one sense we are all sons and daughters of God(Heavenly Father), because He is literally the Father of our spirits, which He created before we came to this earth and obtained our bodies. In our pre-earth life we were all one big family of brother and sister spirit bodies(Moses 3: 5).
Then after our bodies unite with our spirits when we are born, we have the opportunity in a different sense to figuratively become the sons and daughters of Jesus Christ, when our hearts are changed and we make covenants with Him(baptism) and exercize faith in Him:

"But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name:"(John 1:12)

"And now, because of the covenant which ye have made ye shall be called the children of Christ, his sons, and his daughters; for behold, this day he hath spiritually begotten you; for ye say that your hearts are changed through faith on his name; therefore, ye are born of him and have become his sons and his daughters. (Mosiah 5:7-8)

So even though, through God the Father, our spirits are already literally brothers and sisters, we need to also become figuratively Christ's sons and daughters , through faith and obedience to His gospel and baptism, as stated in the Book of Mormon and the Bible(See also Gal 3:26-27).