What should I do about the money my maternal uncle left with me, having instructed me to hand it over to his children in a specific way after his death, given that my uncle has passed away and his children refuse to implement his will?

A will that offers no benefit to anyone, or leads to the rightful owner being deprived of their right, or is intended for control without any genuine interest, is an illegitimate will and is not to be executed.

Thus, the money your maternal uncle gave you, with the condition not to give it to his children unless they are in need, this condition is impermissible. This is because the money transfers as an inheritance to his children immediately upon his death. A mature, adult, sane, and sensible individual is not under anyone's guardianship. Therefore, the rightful owner cannot be prevented from taking their due, nor can one wait for them to need the money before giving it to them. This is because the money is their right, whether they are in need or not. If they are not in need, the money will remain unused until they die, exposing it to loss or depreciation, and all these harms and corruptions are not endorsed by Islamic law.

Similarly, your maternal uncle's condition that this should take place at his grave is a restriction upon his children. What is the benefit of making the deceased a witness? The deceased generally hears the speech of the living, but not always.

Therefore, we advise you to give the money to his eldest son or eldest daughter as he instructed you. By doing so, you will clear your conscience and have fulfilled the trust.