So far as recovering money from defaulters, bank proceed against mortgaged property first, which was also done in your case.
Thereafter if the amount recovered from the sale of the mortgaged property is insufficient, then the bank has the option to proceed against the guarantors of the loan, in this case against your father in law and your wife. If they are owning properties either movable or immovable these can be attached for realization of outstanding amounts.
If your father in law does not own any property then the bank cannot move against the property of his daughter, your sister in law as the property at present vests with her.
Follow the case filed by the bank. Most likely bank will get an order from the court, but eventually this will only be a paper decree, as they cannot recover the monies, and may have to close the loan account as NPA or bad debts, in there books of account.