Are you considered a participant in the sin if you give your brother money and he uses it to buy qat?

The answer is summarized as follows: The ruling on Khat is disputed, and its lowest state is that of doubtful matters. The fatwa on Islamweb is that it is forbidden. Therefore, it is not permissible to give money to a brother if it is known that he will buy Khat with it, as this constitutes aiding in sin. However, money can be given to him for his permissible needs on an as-needed basis.