Well, yes we do have a duty to point out the sin of others. Not to judge it, but to mention it for the good of the sinner. This is an act of charity and love, and love disciplines those whom she loves.
Luke 17:3 So watch yourselves. "If your brother sins, rebuke him, and if he repents, forgive him.
Galatians 6:1 “Brothers, if someone is caught in a sin, you who are spiritual should restore him gently. But watch yourself, or you also may be tempted.”
Note the caution to watch ourselves for the presence of sin, as well as caution in dealing with it in order to avoid that very same sin.
Additionally, the Catechism teaches that sin is a danger to the unity of the Church (CCC 814 & 817) and causes harm to the community of believers (CCC 952). We also have a responsibility for the sins of others if we cooperate with them by participating in them, or by ordering, advising, praising or approving them, as well as by failing to disclose or hindering them when we have an obligation to do so (CCC 1868).