All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
Seeing true dreams is a sign of truthfulness as mentioned in some authentic Ahaadeeth. In a Hadeeth on the authority of Abu Hurayrah the Prophet, sallallaahu ‘alayhi wa sallam, said: “When the time draws near (i.e. near the end of the world), the dream of the Muslim can hardly be false; the most truthful of you in their speech are those who see the truest visions.” [Muslim]
However, this ruling applies to most cases but not to all cases. Hence, it does not mean that the one who sees true dreams never lies. Nevertheless, true dreams are generally a sign of one’s righteousness and abstinence from lying. On the other hand, if your sister lies sometimes, then you should advise her leniently and explain to her the prohibition and danger of lying. Your advice should be made for the Sake of Allaah The Almighty and you should utilize the fact of seeing true dreams to encourage her to adhere to truthfulness. Explain to her that true dreams should increase the Muslim’s adherence to piety and abstinence from lying rather than being a reason for self-conceit.
For more benefit, please refer to Fatwa 19059.
Allaah Knows best.