Is it permissible to sell a car to a brother through the bank, with the seller assisting his brother in paying the monthly installments?

If the bank buys the car from you and then sells it to your brother in installments without a late payment penalty, this is a permissible murabaha transaction, and you will be rewarded for helping your brother with the payments. However, if the transaction between your brother and the bank involves usury, then it is forbidden, and it is not permissible to assist in it.