Cults differ widely from each other, of course, but they often have several characteristics in common. (Your local Christian bookstore can suggest some books that describe cults in more detail.)
One characteristic is that cults reject the basic beliefs of the Christian faith–beliefs that Christians have held in common for almost 2,000 years. Instead, they say they alone have a full understanding of the truth about God, and the only way to know the truth is to be part of their group. Many cults have their own writings also, which they either substitute for the Bible or add to the Bible.
Cults also often have a strong leader–one who demands total obedience, and actually claims to speak for God. This is very dangerous, of course, because he or she may lead others into disaster. Remember: Only Christ is worthy of our allegiance, for only He is God's Son. The Bible says, "Through him you believe in God ... so your faith and hope are in God" (1 Peter 1:21).
Pray for your brother and ask God to help you share Christ's love with him. Cult members are often very resistant to outsiders, but pray that in time he will see this group's false claims. Most of all, may his experience challenge you and your family to a deeper commitment to Christ.