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.
Going to soothsayers or diviners and believing what they say is an act of faithlessness. That is because the Prophet said that whoever went to a soothsayer or a diviner and believed in what he said had no faith in what was sent to Muhammad . [Ahmad, As-Suyooti classified it as Hasan (good) Hadeeth] Thus, it is not permissible to go to soothsayers or to believe in them. Acting according to what he said means you believe in him. Only the person who does this will be sinning.
At the same time, you have to advise your relative and explain to him how dangerous it is to seek the help of soothsayers and diviners, and that most of what they say is lies and delusion. Explain to him that all matters are in the Hands of Allaah The Almighty. That means that Allaah The Almighty knows the condition of your brother and if you are destined to find him, this will definitely happen. Nothing happens except what Allaah The Almighty has decreed and ordained in His prior Knowledge.
Remind him of the story of Yoosuf (Joseph) and how Allaah The Almighty reunited him and his father and brothers after many years. On the other hand, you may continue pursuing the Shar‘i (legislated by Islamic law) means and supplicate Allaah The Almighty much and seek His Help. This can solve this problem, Allaah willing.
Allaah Knows best.