Is it permissible to give Zakat to siblings, even if the Zakat-payer is their provider?

It is permissible to give zakat to needy siblings, even if you are already supporting them, as they have a greater right to it. As for others among those whose father (your father) passed away, if they are people whose support is obligatory upon you, such as your needy mother, then it is not permissible to give them zakat. If they are not in that category, such as your father's wife who is not your mother, then it is permissible.