Is it permissible to help a brother repay the principal of a usurious loan that a guarantor paid on his behalf, given the brother's inability to pay and his need?

There is no harm in helping your brother repay the principal of his usurious loan, with him or his guarantor paying the excess beyond the principal. Everyone should be advised to repent and not return to this forbidden transaction, as it is among the major sins and a defiance of Allah. Allah Almighty said: "O you who have believed, fear Allah and give up what remains [due to you] of interest, if you should be believers. And if you do not, then be informed of a war [against you] from Allah and His Messenger. But if you repent, you may have your principal – [thus] you do no wrong, nor are you wronged." And the Messenger of Allah, peace and blessings be upon him, cursed the one who consumes usury, the one who feeds it, its scribe, and its two witnesses.