Is the stepdaughter considered sinful if the papers for a usurious bank loan to buy a house are in her name, given that the father's wife is the actual debtor and payer of the loan?

Borrowing with interest is Islamically forbidden, and it is one of the major sins and the seven destructive sins. It is not permissible to undertake it except out of necessity that permits committing the prohibited. Therefore, it is not permissible for your father's wife to borrow with interest, and it is not permissible for you to assist her in that, based on the Almighty's saying: "And do not cooperate in sin and aggression," and based on the Prophet's (peace be upon him) saying: "The Messenger of Allah (peace be upon him) cursed the one who consumes Riba (interest), the one who pays it, the one who records it, and its two witnesses, and he said: 'They are all alike.'" There are legitimate alternatives that obviate the need to resort to Riba and forbidden transactions.