What is the ruling on money borrowed with interest from a financial institution to complete the furnishing of an apartment with money inherited by the brother from his father?

Borrowing with interest (riba) is forbidden and is considered one of the major sins. It is not permissible for your brother to take your money and your sister's money without your consent. Your assistance to your brother in furnishing his apartment is a righteous deed for you, as it is an act of maintaining kinship ties, helping him abandon disobedience (borrowing with interest), and assisting him in obedience (marriage).