Does sin befall the wife if her husband buys a new house with an interest-based loan, relying on a fatwa that permits it, knowing that he already owns a house also bought with an interest-based loan, and he intends to rent out the garage of the new house to pay off the bank installments?

Borrowing with interest is one of the greatest forbidden acts, and it is not permissible to undertake it except out of necessity. There is no sin upon you if you advised your husband to abandon interest-based borrowing and he did not heed the advice. Nor will you incur sin by residing in the dwelling he purchases with an interest-based loan; because the prohibition in an interest-based loan pertains to the borrower's liability, not to the money itself.