I am so sorry for your loss and for the problems that have resulted.  If you are the named Executor in the Will then you have the right to bring what ever law suits on behalf of the estate that you think are necessary.  If your Father had a right to sue then his estate has the right to sue (and you would bring suh a suit).  Now, I know that these are very difficult times.  Things between siblibgs get emotional and heated. I think that your suggestion about asking her to return the funds is good. If she does not then it may be a good idea for you to speak with an attorney about writing a letter to your sister for an accounting of the funds and requesting that she reimburse the estate.  If she does not then decide if a suit is the way to go.   Good luck.