What obligation, if any, do I have to family or friends to educate them on Catholic matters? For instance, if my sister doesn't know the Church's teaching on birth control, do I have any obligation to tell her? Or regarding a brother who is living with his girlfriend, do I have to tell him that what he is doing is immoral? If I do say something, is that judging?
We make judgments all the time. Should I trust this person with my child? Is this person telling me the truth? Is the person who said he’s sorry sincere? We judge what is right and what is wrong. What we must not judge is the condition of another’s soul or its destination. That is because an act that concerns an objective evil is only one condition of mortal sin. There must also be full knowledge and full consent; otherwise the culpability for the sin may be diminished. Only God knows for sure.
"Indeed love itself impels the disciples of Christ to speak the saving truth to all men. But it is necessary to distinguish between error, which always merits repudiation, and the person in error, who never loses the dignity of being a person even when he is flawed by false or inadequate religious notions. God alone is the judge and searcher of hearts; for that reason he forbids us to make judgments about the internal guilt of anyone" (Gaudium et Spes 28:2).
It is never permissible to leave a someone in a state of ignorance concerning grave matters of faith and morals, for Christ commanded us to preach the gospel to all creatures and to teach them "to observe all that I have commanded you" (Matt. 28:20). Prudence requires, though, that you choose the appropriate manner, time, and place.
Although it is normal to feel intimidated or uncomfortable when you’re talking about a sensitive moral issue, don’t let a little fear and intimidation stop you from speaking the truth. You have to witness to the truth for the sake of the other. Truth is the most loving gift you can give to anyone. To prepare for such discussions, continue to grow in your understanding of the Church’s teachings so you can witness more effectively—and pray. Pope Paul VI gives this exhortation in Evangelii Nuntiandi: "Every evangelizer is expected to have a reverence for truth. . . . He never betrays or hides truth out of a desire to please men, in order to astonish or to shock, nor for the sake of originality or a desire to make an impression. He does not refuse truth. He does not obscure revealed truth by being too idle to search for it, or for the same of his own comfort, or out of fear" (EN 78).
He then asks every evangelizer "to pray about the following thought: Men can gain salvation also in other ways, by God’s mercy, even though we do not preach the gospel to them; but as for us, can we gain salvation if through negligence or fear or shame—what St. Paul called ‘blushing for the gospel’—or as a result of false ideas we fail to preach it? For that would be to betray the call of God, who wishes the seed to bear fruit through the voice of the ministers of the gospel; and it will depend on us whether this grows into trees and produces its full fruit" (ibid., 80).