In Paul's time, the old was a separation of the people from God. Priests were the communicators between God and His people. Sin separated people from God. With Jesus came the news, Sins are forgiven. We are in a new, personal relationship with God. We no longer need priests to speak for us. We can speak to God on our own. With this also comes the Way outlined by Jesus. Love one another as he loves. And love God.
Sins are forgiven. We are a new creation in the ways of Christ.
Some believe they have no sins worth noting. They have nothing to turn from, and perhaps you feel this. Jesus noted that he did not come for the healthy, but for those who are lost. Perhaps this is how your sister felt...that she had been lost, but Christ found her and introduced her to new standards, new ways.