Is it permissible to consider money sent to the father who supports the siblings in their education, or a portion thereof (to the extent of the siblings' benefit), as part of the Zakat al-Mal paid at the end of the year?

The money spent by the questioner in the past to help his father and brothers cannot be counted as part of his Zakat, because the intention [of Zakat] was not present when it was given.

As for the future, it is not permissible to give Zakat to the father for his essential expenses unless the son is unable to support him. It is permissible to give Zakat to the father to pay off his debt.

As for the brothers, it is permissible to give them Zakat if the father is unable to support them, and they themselves are unable to earn a living. It is necessary to transfer ownership of the Zakat to them if they are adult and mature. If they are not adults, the Zakat should be given to whoever is responsible for their maintenance, to spend it on their behalf for their welfare.