What is your father's opinion about this?
You say so much but you have told anything about your father's vies on this.
If your father had agreed to take that amount he only has to pressurise his sister to get that money in his own way and method.
Since this is oral agreement and no documentary evidence, if your father is planning to initiate any legal action on this, he may stand losing the benefits what was assured to him.
Hence this needs diplomatic thinking and careful handling.
Work out the strategies properly and bash your father's sister through counterblast action unless they will not give in to the pressures.