Have I committed a sin by lying to my sister about a dream in which I saw my deceased father asking her to end her relationship with a young man, in order to make her break off an unlawful relationship, and must I tell her the truth?

Claiming to have seen a vision one has not seen is a grave sin and a major transgression (kabirah), exposing the doer to punishment on the Day of Judgment. Lying about a vision is worse than lying in wakefulness because it is considered a lie against Allah, for a truthful vision is a part of prophecy. It is impermissible to lie in a dream, even if it is for a perceived benefit, because the call to Allah cannot be through disobedience to Him. Whoever has done this must make sincere repentance and should not inform the person they lied to, in order to avoid corruption, and they should remind her of Allah.