As you are already doing what you can to witness to your sister positively, while maintaining a close relationship that may serve as a channel of grace for your sister, you are doing what you have determined according to the best of your prudential judgment to be in your sister’s best interests.
If I were you, I would be more concerned about the friend who uses the apostle Paul as a pretext for suggesting the destruction of your relationship with your sister. The suggestion that your continued contact with your sister constitutes “walk[ing] with her to hell” not only presumes to judge your sister’s eternal destiny but also your own. Only God can determine your sister’s level of personal culpability for objectively grave actions; only God can know that you yourself are acting according to your own best judgment of what is right. While your friend might do things differently were the situation his or hers to deal with, that person should not presume to render judgment on your or your sister’s internal dispositions that were involved in determining the choices each of you made.
My suggestion would be to ignore the advice your friend gave and continue to do your best to be a positive influence in your sister’s life. Should your friend presume to demand to know if you’ve followed through on the advice, you might tell your friend that you’ve decided that Paul’s exhortation on love (cf. 1 Cor. 13) requires you to address the situation in a different way.
Love is patient and kind; love is not jealous or boastful; it is not arrogant or rude. Love does not insist on its own way; it is not irritable or resentful; it does not rejoice at wrong, but rejoices in the right. Love bears all things, believes all things, hopes all things, endures all things. … For now we see in a mirror dimly, but then face to face. Now I know in part; then I shall understand fully, even as I have been fully understood. So faith, hope, love abide, these three; but the greatest of these is love (1 Cor. 13:4-7, 12-13).
To avoid rash judgment, everyone should be careful to interpret insofar as possible his neighbor’s thoughts, words, and deeds in a favorable way: Every good Christian ought to be more ready to give a favorable interpretation to another’s statement than to condemn it. But if he cannot do so, let him ask how the other understands it. And if the latter understands it badly, let the former correct him with love. If that does not suffice, let the Christian try all suitable ways to bring the other to a correct interpretation so that he may be saved (CCC 2478).
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