Must I obey my father's wife as I obey my mother, given that she harms me and deprives me of my father's money, which he has started to give in to her demands?

Allah has made obedience to the mother obligatory in what is good, not obedience to the stepmother. You are not religiously obligated to obey your stepmother. We advise you to be patient with her harm. She has no right to prevent you from receiving good from your father. If the expense is obligatory, then her prevention of it is a sin. Advise your father and remind him of Allah, and be kind to your stepmother, for your kindness to her is part of your dutifulness to your father.