Is it permissible in Sharia for a son to ask his father to suffice with 100,000 EGP as his contribution to securing an apartment for his brother, due to his personal expenses, marriage costs, and feeling the pressures of living abroad, while fearing that he might be disobedient to his father?

You are not obligated to spend on your brother because you are not his heir. Therefore, you are not bound to obey your father in this matter, nor are you considered disobedient. Your father's condition that you help buy an apartment for your brother in exchange for giving you an apartment is considered a gift of reward. As for your contribution to your father's expenses, it is obligatory if he is in need of it. We advise you to please your father by contributing to the house and your brother's expenses, as this is an act of righteousness and benevolence.