I'm thankful for your sister's concern for you and for your concern, as well. And let me assure you that God doesn't want you to be led astray, either. He wants you to come to know Him and walk with Him every day.
How can you know if this group (which you don't name) is a cult? First, cults almost always claim that they, and they alone, have the truth. They turn their backs on almost 2,000 years of Christian history, and say that other churches are wrong and will deceive you. They point instead to their leader or founder as the final authority, and claim that God has given them a new revelation of Himself.
Second, cults always reject what the Bible teaches about itself. The Bible alone is God's Word but cults add to it or substitute other books in its place. One cult even insists that its "translation" of the Bible is the only reliable one in spite of the fact scholars universally reject it.
Most of all, cults reject Jesus Christ as God's way of salvation. They may reject His full divinity, or claim that faith in Him is not enough to save us. But Jesus said, "I am the way and the truth and the life. No one comes to the Father except through me" (John 14:6). Don't be deceived, but turn in faith to Christ for your salvation and ask Him to come into your life. Then ask Him to lead you to a church with Christ at its center.