Your mother was sure she was going to heaven for one reason: She had put her faith and trust in Jesus Christ to save her—and that made all the difference.
Let me explain it this way. If you stop and think about it, you will realize that there are only two ways we could ever get to heaven. One way would be for us to earn our salvation—that is, for us to be so good that God would welcome us in to heaven.
But how good would you have to be? The answer may startle you: You'd have to be perfect! The reason is because God is holy and pure, and He cannot tolerate sin of any kind. That's why even one sin would be enough to keep you out of heaven. The Bible says, "For whoever keeps the whole law and yet stumbles at just one point is guilty of breaking all of it" (James 2:10).
Is there any hope? Yes—and the reason is because God has given us another way! That way is Christ. He was God's only Son, and because of that He was without sin. But when He died on the cross, all our sins were placed on Him, and He took the punishment we deserve. God loves you that much! The Bible says, "This is love: not that we loved God, but that he loved us and sent his Son as an atoning sacrifice for our sins" (1 John 4:10). This is the truth your mother knew—and you can know it too, by giving your life to Jesus Christ and trusting Him alone for your salvation.