I am not an attorney and never was. My "guess" on the POA's vs the attorney as named Trustee of the revocable trust, is that the attorney would have the stronger position.
That "Durable Power of Attorney"---do the documents say anything as to what types of your father's affairs it applies to? The one thing I am again "guessing" about is "maybe", with POA's, you and your sister could "revoke" the revocable trust your father set up. That would leave that other attorney as trustee of nothing.
With five children involved and two of them you describe as "difficult", I can understand why the attorney named as trustee is leary to do anything without full consent of all five.
In the end, without all five children being on the same wavelength, it sounds like a court will have to untangle the mess.
Dreams Worth Dreaming are Dreams Worth Planning For. I Spent a Career Planning for Early Retirement.