Is going to a place that a soothsayer predicted about my missing brother considered a sin upon us, or does the sin fall only upon the one who went to the soothsayer?

Going to a soothsayer or diviner and believing him is an act of disbelief (kufr), because the Prophet, peace and blessings be upon him, said: "Whoever goes to a soothsayer or diviner and believes what he says, has disbelieved in what was revealed to Muhammad, peace and blessings be upon him." Therefore, it is not permissible to go to a diviner, believe him, or act upon his words. The sin falls only on the doer, but you must advise your brother and clarify the danger of what he is doing, and that what soothsayers say is lies and deception, and that all matters are in the hands of Allah. You must continue to take lawful measures, make abundant supplications, and resort to Allah to solve the problem.