Am I permitted to promote an Islamic forex company in exchange for a percentage of the profits of those who register through me, even though my brother believes that I would bear the sin of those among them who do not adhere to Sharia guidelines?

If the company adheres to Sharia compliance in its financial dealings, then there is no harm in dealing with it and attracting customers to it. However, if it deals with the customer according to his desire, then whoever you know or strongly believe desires the unlawful, it is not permissible to direct him. As for whoever you know desires the lawful, or whose matter you are unaware of, the original ruling is that it is permissible to direct him, and the sin is on him if he chooses the unlawful.