All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
There is nothing wrong with helping your parents-in-law by speeding up the payment of the interest-based loan, especially if by speeding up the repayment, the Riba (interest) will be removed from them, but they should seek the forgiveness of Allaah for what they did and be determined not to repeat it.
They are entitled to benefit from their home that they bought with that loan because the prohibition of the interest-based loan does not relate to the thing on which the money was spent but, rather, to the person who took the loan.
Allaah knows best.