Is the sister who withdrew her money before the agreed-upon period entitled to profits for her period of participation, based on her claim of a promise from her deceased brother, or is she not entitled to anything according to the original condition?

The fundamental principle is that a partnership contract is permissible and not binding, and a partner may terminate it at any time. Jurists have differed regarding the stipulation of a specific duration for a partnership or Mudarabah (profit-sharing partnership); the Hanafis and Hanbalis permitted it, while the Malikis and Shafi'is prohibited it.

The Islamic Fiqh Academy holds the view that there is no religious impediment to stipulating a fixed term for Mudarabah if both parties agree. In such a case, it becomes binding until the end of the term, and neither party has the right to terminate it except by mutual agreement. However, if the partners agree to terminate, it is impermissible to stipulate that anyone who withdraws from the partnership before the end of the term forfeits their share of the profit, as this constitutes gharar (uncertainty/deception) and wrongful consumption of wealth. Rather, the profit should be assessed at the time of their withdrawal, and they should be given their share. This can be calculated in two ways: either by considering the concluded transactions and giving them their share of the realized profit, or by calculating the profit based on the duration of their participation. It is permissible to use the "Nimr" (tiger) method for distributing profits, which considers each investor's capital and the duration of their investment.

Therefore, the sister must be given her share of the profit, whether the brother promised her so or not.