Before you spend your money, I hope that you have more to go on than what you have stated here. Other than your pique concerning the trust your father has in his grand niece and him essentially telling you, "It's my decision; get over it", I just don't see that the paranoia is justified.
And... Cases of executors stealing are very rare. Bad decisions sometimes... sure. Decisions with which beneficiaries disagree... yeah. Actions - intentional or not - that benefit one beneficiary over another... sometimes. And so on.
You just aren't going to read about the 99+% cases where the executor fulfills his/her fiduciary obligation completely and everybody comes away happy.
If you go forward, what could happen is:
1) You spend your money on attorney and don't accomplish anything.
2) The grand niece decides to retain the most expensive probate attorney in Idaho to represent her as the executor, with said attorney's compensation being paid from the probate estate funds.
3) The grand niece is peeved enough at essentially being called a potential thief that she renounces that "no compensation" provision in the will and takes as much compensation as the probate court considers reasonable.
4) Your sister gets royally ticked when she realizes how much additional expense and time has been incurred.