Loan against the property which has been divided

Dear sir, My grand father had two wive's 1 st wife had 2 children (son) ,second wife alive and she has 1 son & 3 daughters ,My grand father had property, and he has divided his share to three of his sons and stamped. in this 2 property can take after his death (he mentioned till his life time can use the property) but few years back he sold one of the property with out intimation of two of his son (1 st wife son) due to 2nd wife & her son & daughters forced for money so he sold but my father& his brother asked no proper reply so he put lawyer notice later he convinced and he has give some amount of money as well as we took as a writing statement from him hereafter will not be take loan against the property & sales too ,now he has written one of his share to all children (girl child also with out our knowledge - How is it possible, it is already written and stamped in register office? ) and he is going to take loan against the property now ,he is asking sign from me (my father was dead 4 yrs back) and my mom & younger father we refused because later he get a loan will sold the property this condition what can i do ,kindly help me