Is it permissible to give a single dinar in charity out of five dinars, despite being in dire need of them oneself or one's siblings being in need of them, or should the needs of the siblings be prioritized?

If the siblings are in need, then giving charity to them is better and carries a greater reward than giving it to another poor person. This is based on the Prophet, peace and blessings be upon him, who said: "Charity given to a poor person is a charity, and to a relative, it is two: charity and maintaining kinship ties." Thus, giving charity to relatives yields a multiplied reward.