My brother didn't tell my father and sister the truth. And I was there knowing the truth . Should i have said the truth and expose my brother lying?

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 is His Slave and Messenger.

It is better to advise your brother and try to guide him to stop lying by reminding him of the Allaah's punishment for liars. The Prophet said:

“Keep to truth, for truth leads to righteousness and righteousness leads to Paradise, and if a man continues to speak truth and pursue truth, it will be written with Allaah that he is truthful. And beware of falsehood, for falsehood leads to wickedness and wickedness leads to the fire, and if a man continues to speak falsehood and pursues falsehood, it will be written with Allaah that he is a liar.” [Al-Bukhari and Muslim]

We do not believe that exposing your brother's lying to your father and sister would solve anything except if it was absolutely the only way to stop him from committing that prohibited act or if not doing so would entail allowing the rights of your father and sister to be compromised as a result of his lies, and as long as doing so (exposing him) would not cause graver evil.

Allaah Knows best.