Is it permissible to give a portion of Zakat to a spendthrift brother who works as a painter and has a family, knowing that his circumstances are below average?

If your brother meets the criteria of being poor or needy, then it is permissible to give him zakat, and he has a greater right to it than others; because charity given to a relative is both charity and maintaining kinship ties, as the Prophet, peace and blessings be upon him, said: "Charity given to a poor person is charity, and charity given to a relative is two: charity and maintaining kinship ties."