1. Since your brother in law has taken loan after mortgaging the property of your father in law, the said mortaged property can not be dealt with by your father in law now without the written approval of the mortgagee Bank.
2. Since your brother in law is not in a position to pay the EMIs, all of you being your brother in law, your father in law and yourself should file a joint application before the said mortgagee Bank for granting permission to your father in law to sell the said mortgaged property to you, sale deed for which shall be kept with the said Bank by you as continuation of its mortgage and also to transfer the said loan in your name, EMIs for which you shall repay.
3. This way the Borrower being your brother in law will be substituted by you in the records of the Bank and the property which will be kept under mortgage by the Bank will stand in your name.
4. If the above steps are taken, your brother in law will not be in a legal position to claim the said property in any way later on.