I don’t doubt your sister’s sincerity, but simply emptying our minds of distractions and experiencing feelings of peace as a result isn’t the same as knowing God.
Let me explain. The Bible says that God created us, and He wants us to know Him and experience His presence every day. But something has cut us off from God, and that “something” is what the Bible calls sin. God is pure and holy, and because we have ignored Him and gone our own way, we have become separated from Him. The Bible puts it this way: “Your iniquities have separated you from your God; your sins have hidden his face from you” (Isaiah 59:2).
How, then, can this barrier of sin be removed? We can’t remove it ourselves, no matter how hard we try. Our good works won’t remove it, and neither will the type of meditation your sister practices. Only God can remove it — and this is what He did by sending His only Son, Jesus Christ, into the world to atone for our sins by His death. And when we turn in faith and trust to Him, God promises to forgive us and make us His children forever.
Encourage your sister to open her heart and mind to Jesus Christ. But no matter what she chooses to do, my prayer is that you will discover the joy of God’s peace by committing your life to Jesus. The Bible says, “For he himself is our peace, who… has destroyed the barrier” (Ephesians 2:14).