Is it permissible for a participant in a usurious loan to repay only his share of the principal, or must he also pay his share of the usurious interest?

You must advise your brother to abandon usurious borrowing and repent. However, if the borrowing has already occurred, there is no harm in dealing with him regarding the borrowed money in a permissible manner, because the money itself is not forbidden; rather, the prohibition is related to your brother’s obligation.

Accordingly, it is permissible for you to borrow from your brother if it is without a condition of increase in repayment. However, if it is with a condition of increase, it is forbidden usury and one of the major sins.