Is it permissible for a person who is responsible for the care of his minor siblings and spends on them from their money, to disburse these funds in a manner not divided among them, and are his actions considered correct?

You have no guardianship over your adult, mature siblings regarding their wealth, and they have the right to dispose of it as they wish, except with their consent. You can offer advice and oversee their spending and the saving of what remains for them. As for your younger brother, if he is not an adult, you have no guardianship over his wealth except by a will from your father or by a judge appointing you as his guardian. In this case, it is permissible for you to mix your money with his and take what is sufficient for you from it in a reasonable manner if you are in need.