Is what I am doing—taking money for my efforts in managing my father's business without his knowledge—considered permissible (halal) or forbidden (haram), knowing that my siblings do not share in this effort, and my father refused to set a wage for me, saying, "It is my money"?

It is not permissible to take money from the father without his knowledge on the pretext of extra effort, and it is obligatory to return these monies. If the son wants payment for his work, he must inform his father, and whatever the father approves of and gives to him is lawful for him. The father's statement: "This is your money" does not permit taking it without his knowledge.