Is it permissible for a person to say to his sister, "You are a pure soul," as a compliment, and phrase it in a context that brings her closer to obedience to Allah?

If someone is told, "Your soul is pure (نفسك طاهرة)," there is no harm in that if what is meant is its purity from polytheism and immoralities, or the purity of his body, because a believer is not impure. However, if there is a fear that the praised person might become arrogant or conceited, then praising him is not permissible, due to the Prophet's (peace be upon him) prohibition of praising someone to his face, and because it may lead to lying, hypocrisy, or hardening of the heart. But if there is no fear of temptation and there is a legitimate benefit, such as refuting an accusation or strengthening the heart of someone whose faith is weak, then praise is permissible at that time, such as being gentle with disobedient people to win their hearts and encourage them to accept advice.