If elder brother has taken loan against the property and have mortgaged his property, then his legal heir is required to pay the loan in order to claim the rights in the property. Since as mentioned by you that elder brother has no legal heir as in wife or son, then as per Hindu Succession Act, the property will devolve to his mother and if mother is not there then it shall devolve to all legal heirs.
Therefore, the other sibling will have to settle the amount with the mortgagor to claim the rights on the property. The other sibling should pay the loan of elder brother and execute the settlement agreement.