Yes, it’s true that some religious cults deliberately seek out people who are easily influenced—either because they have no real convictions of their own, or because they aren’t emotionally strong enough to resist pressure from others.
This doesn’t mean everyone who becomes involved in a cult is this way, of course—but it does mean some people are especially vulnerable to a cult’s appeal. In fact, one characteristic of many cults is that they are led by a strong leader who dominates its members and demands their total loyalty. Leaders who seek to serve Christ, however, aren’t this way, but are “eager to serve; not lording it over those entrusted to you” (1 Peter 5:2-3).
What can you do to help your sister? First, examine your own spiritual life. Have you ever decisively committed your life to Jesus Christ, and are you seeking to follow Him and live for Him every day? Make sure of your commitment to Christ, and then ask God to make you an example to your sister of His love and peace and joy. She may not be open to what you tell her—but she can’t argue with the reality of a life that’s been changed by Christ’s power.
Then pray for your sister. Pray that God will show her that she’s on the wrong path, and will give her the courage to turn to Jesus. Pray too that God will give you a deeper love for her, and will give you opportunities to talk with her about Christ.