Is it permissible for a man to propose to a woman already proposed to by his brother, after the brother of the proposed woman has refused (the first proposal), and is he who proposes to her thereafter considered a hypocrite despite his commitment to helping me first?

If the girl accepts you and does not withdraw her acceptance, it is not permissible for anyone to propose to her, based on the saying of the Prophet, peace and blessings be upon him: "A man should not propose over the proposal of his brother until he marries or gives up." The one whose acceptance or rejection is considered is the girl herself, as long as she is an adult and of sound mind, or her guardian if she is compelled. As for the man who proposed on behalf of his son, it is obligatory to think well of him; he might have thought that she had withdrawn her acceptance of you. It is not permissible to force an adult, sane woman to marry someone she does not desire, nor is it permissible for her guardian to prevent her from marrying a suitable man she wishes to marry; otherwise, he would be an oppressor, and she has the right to take the matter to a judge.