Is it permissible to borrow from relatives who deposit their money in a usurious bank, and pay them a monthly return instead of the bank, or what is the legitimate Shar'i way for a permissible contract that secures a monthly income for them?

Borrowing money from relatives on the condition of returning it with a monthly increase is forbidden interest (riba), one of the gravest major sins and destructive acts. Scholars have unanimously agreed that any loan which stipulates an increase or benefit for the lender is usury (riba). The permissible way is a goodly loan (qard hasan (definition)) without any increase, or partnering with them in purchasing the land through a sharecropping contract (muzara'ah), or a Mudarabah (definition) contract with their money, provided that the profit is shared between you according to the agreement.