Have you asked your sister to explain what has happened to her? Admittedly, she may have trouble putting this into words, but from what you say, it sounds like she has come to know God in a deeply personal way, by giving her life to Jesus Christ.
You see, our greatest problem is that we are separated from God, and the reason is because we are sinners. We may not be terrible people as far as others are concerned, but even one sin is enough to separate us from God. How, then, can this stain of sin be erased?
The Bible says we can’t do it on our own, because even when we do our best, the stain of sin is still there. As the Bible says, “There is no one righteous, not even one” (Romans 3:10).
Only God can forgive us and cleanse us from our sins, and he made this possible by sending his son, Jesus Christ, into the world. He was without sin (for he was God) — but when he died on the cross, all our sins were transferred to him, and he became the final and complete sacrifice for our sins. And when we put our faith and trust in him, God promises to forgive us and give us the gift of eternal life.
Don’t trust in your good works, or anything else, for your salvation. Instead, do as your sister has done, and put your faith and trust in Christ. The Bible’s promise is true: “There is now no condemnation for those who are in Christ Jesus” (Romans 8:1).