Is it permissible to take an additional percentage of the profit without the brothers' knowledge, in exchange for guaranteeing the capital, knowing that the remaining profit is satisfactory for them?

A Muslim must learn the rulings of transactions before engaging in them, and contracts must be adhered to, in accordance with the Almighty’s saying: {O you who have believed, do not consume one another’s property unlawfully but only [in lawful] business by mutual consent}, and His saying: {O you who have believed, fulfill [all] contracts}.

The transaction mentioned is Mudarabah (profit-sharing partnership), where one of you provides the capital and the other provides the labor. You share the agreed-upon profit, and the loss is borne by the capital. This is permissible by consensus. However, your inclusion of your siblings' money in the Mudarabah without agreeing on a specific fee or percentage makes you a volunteer agent. It is not permissible for you to take a percentage of their profits without their knowledge and consent. However, you may request a known fee for your work. It is not permissible to stipulate a guarantee, pledge, or a guarantee of the principal or profit in contracts of trust (amanah) and Mudarabah. If a guarantee is stipulated, the condition is void but the contract remains valid. A Muslim must be clear and explicit in their dealings and specify the compensation in advance. It is preferable that it be a fixed amount to avoid dispute regarding the permissibility of a percentage of the profit as a fee.