Is it permissible for an agent to establish an endowment (waqf) for his father without the knowledge of his mandating siblings?

An agent is not permitted to dispose of his principal's property except within the limits of what he has been authorized to do; otherwise, he would be exceeding his authority and held liable. Therefore, you are not allowed to create an endowment (waqf) on behalf of your father from your siblings' money without their permission. However, it is permissible if they grant permission or if it is from your own private funds. This act is considered a form of filial piety towards your deceased father, and he will receive its reward.