What is the ruling on a brother buying a car, then selling it to the questioner through an Islamic bank via a Murabaha (cost-plus financing) system, so that the brother can recover the debt he lent to the questioner?

It is not permissible to borrow from a bank with interest (a surplus), as this is usury (riba), which is forbidden by Islamic law. The car in this case is a trick that does not change the reality of the usurious transaction. This is what Ibn Abbas expressed as "dirhams for dirhams with a piece of silk in between," and this falls under the 'inah sale (buy-back sale) which is prohibited by Islamic law. A permissible tawarruq transaction can be conducted, whereby the bank buys a car or another commodity other than your car, then sells it to you for you to take possession of it, sell it, and repay your debt with its price.