Joel Osteen is dead wrong on calling Mormons “brothers in Christ.” This is false teaching because it confuses the gospel and the definition of a Christian. If Mormonism does not teach the true gospel message (which it does not), then those who believe what Mormons teach are not truly united to Jesus Christ. Therefore, they are not saved from sin, death, and God’s judgment to come. It’s that serious.
You can read why in this helpful post (with links to further resources) at The Gospel Coalition.