Is it permissible to take the money promised by the husband's father, knowing that he took out an interest-based loan, and what is the optimal course of action to deal with this situation?

What your husband's father and his brother did by borrowing with interest is a grave sin, and they must repent sincerely. As for the issue of repaying the usurious loan, it is obligatory to repay only the principal without the interest. If it is not possible to avoid the interest and they are compelled to pay it, then they are not required to repay it quickly. As for helping your husband's father, accepting his gift, and borrowing from him, this is permissible, because once a loan is taken, it becomes the responsibility of the borrower and becomes a debt owed by him.