What is the ruling on the money remaining with the brother given his inability to repay, and is zakat obligatory on the money if he returns it, and are the questioner and her brother sinful?

If the salary after the husband's death was due to the deceased, it is part of the inheritance and is divided among the legal heirs according to Islamic law. If it was a donation from the dispensing entity, it should be disbursed according to what that entity specified.

The mother has no guardianship over the orphans' money after the father's death, unless she was appointed as a guardian or in the narrowest of cases where there is no guardian or judge. In such cases, she should act in their best interest.

If the mother disposed of the orphans' money without right, she has erred. If she disposed of it rightfully, there is no blame upon her.

The partner must return the orphans' money unless the money was lost in trade before the dissolution of the partnership in a way that does not guarantee its return. If the loss occurred after agreeing to end the partnership, then he must return all the money.

Zakat is obligatory on the orphans' money if each of their shares reaches the nisab (minimum threshold) and a hawl (full lunar year) has passed on it. Its amount is a quarter of a tenth (2.5%). It is advisable to consult scholars for details regarding the partnership and the management of the money.