Is it permissible to give an indebted brother from zakat money, and is it permissible to collect the zakat of siblings to give to him if the individual zakat amount is not enough to cover his entire debt?

It is permissible to give zakat to a brother who is indebted and unable to repay his debt, because he falls under the category of "al-gharimin" (those in debt) mentioned in the verse about the categories of zakat recipients. A gharim is a debtor who cannot find the means to repay a debt he incurred for his own benefit. Giving zakat to him is more preferable, based on the Prophet's (peace be upon him) saying: "Charity given to the poor is one charity, and charity given to a relative is two: charity and maintaining kinship ties."