Is money earned from working in a shop that sells immodest women's clothing—given the inability to wear the hijab—considered unlawful money, especially if there is an urgent need for this money to help a brother establish a project?

The hijab is an obligation upon the Muslim woman, and it is not permissible to abandon it for work except out of necessity. A woman wearing the hijab does not make her earnings unlawful if the field of work is permissible. It is not permissible to work in selling unlawful women's clothing, as it falls under aiding in sin and transgression. Helping one's brother is a good deed, but it does not permit committing what is forbidden. "And whoever fears Allah - He will make for him a way out and will provide for him from where he does not expect." (Quran 65:2-3). And whoever abandons something for the sake of Allah, Allah will compensate him with something better.