1. Do note that on death of your mother all of you inherited undivided 1/5th share in her proeprty. So later on you can make your father sole owner only on the basis of affidavit or other documents.
2.In other words even though the Khata changed to your fathers name your father did not become its sole owner and all of you continue to have your respective 1/5th undivided sahre in the proeprty.
3.Therefore the Will made by your father can not make sole owner of the proeprty. At the most you are now its 2/5th owner as your father can not bequeath more than his 1/5th sahre.
4. SO if you mortgage your proeprty to obtain loan you would require permission of your sisters.
5. If you want to become the sole owner of the property then you are required to get the 1/5th sahre from your 3 sisters by way of gift deed or deed of relinquishment.