One of the characteristics of most cults (which is what this group seems to be) is that they claim they alone know the truth about God, and they alone can bring a person into contact with Him. And they will do everything they can to convince their members that their claims are true (although in reality they aren't).
Cults reject, therefore, what Christians have believed for almost 2,000 years, and substitute instead their own ideas and philosophies. They may claim they believe the Bible, or they may reject it entirely -- but the result is the same: They deny Jesus was the unique, divine Son of God who came from heaven to save us from our sins by His death and resurrection. Nor do they believe the Bible's clear teaching that we are saved only as we put our faith and trust in Christ.
You may not be able to convince your sister that she's wrong; people in cults often have very closed minds. But let me suggest two things you can do (in addition to praying for her). First, let her see your life. Is Christ's joy and peace evident in the way you live? If so, God can use you to show her that Christ is real and can change her life.
Then let her see your love. Jesus said, "By this all men will know that you are my disciples, if you love one another" (John 13:35). No matter how strong the barriers in her life may be, Christ's love is stronger.