That’s cause in these times most think they are deserving. And when money involved greed takes over.

My parents had a joint will executed in NM. When my mother experienced health issues they moved to WA where my brother, who resided there, had found them a wonderful living facility. My mother soon passed away and her interest in their estate went to my father. My brother took my father to a local attorney to look over the will and make sure all was well with his intentions to pass his estate 50-50 to my brother and me. Somehow, he signed a codicil naming my brother primary executor. Maybe the attorney suggested this as I was not a resident of WA. I really don't know. Dad was 89 years old at the time and in good shape mentally for that age but after a few days of going over the document he had signed he became quite upset and called me to tell me that was not what he wanted to do. He wanted me as the primary executor. He asked me to call the attorney which I did, a very nice lady, by the way. She told me he would have to come in and tell her that he wanted me as the primary and sign a new codicil which of course I knew he had to do. Anyway, they got it done. I called the attorney afterward to verify and asked her if he had made himself clear. She responded "He made himself PERFECTLY clear!" lol

A good attorney is crucial. We went to one that was highly recommended and he cut so many corners that we had to go back 3 times to get our trusts in order. After the third visit for him to correct his mistakes he sent us another bill, I called and told him that his bill was exactly the same as the bill we were sending him for making 3 more trips to his office because of his negligence. Case closed.

Attorney fees for settling an estate aren't cheap. I would approach your dad about how poorly your mom's estate was handled (examples) and suggest he change his will for multiple executors (all you siblings) and wording that requires agreement BEFORE any asset dispersion. My father was specific with some things and more general with others, but it all starts with an appraisal of the estate at the time of death. After your father's specific bequeaths are handled, suggest a round-robin selection where heirs select items based on their $ share of the estate and the appraised value of the items selected. That way the $ value distribution was equal. In our case, if two couldn't agree on who would get an item, it went for auction, and they bided against each other for it. As is, worst case you would have to sue the executor for violating their fiduciary responsibilities, and given their past history, should be a slam dunk.

My brother was Executor when my father passed in Colorado and basically stole everything. Colorado law is in favor of the Executor. My brother was named Executor when my mother passed in California and again tried to steal everything giving things away to his children who showed up with a uhaul truck before I flew out. California requires the Executor to be a fiduciary. I contacted the probate lawyer stating that he broke the terms of the Will and that I would have him arrested if he didn't stop him and pay me my full share. The probate attorney spoke with him, my brother paid me fully for what he had given away, showed me everything financial after that and was told if he tried anything again he would be arrested. Things went well after that, it was a quite large estate. That was 4 years ago. He broke off all contact with me as did his wife and family, but I'm fine with that. Any sibling who would so easily steal from you and totally disregard the wishes of their parent is not anyone I would care to have in my life. Check the laws and rules for Executors in your state and then do what you have to do.

Deaths without perfect wills and executors often generate family animosity. After my grandfather died, my father stopped taking to one of his brothers over who should get what from the estate. Estates often ruin family relationships.

You might consider filing for Guardianship of your Mom in order to stop the financial decline. You should prevail easily with much less legal costs, if he’s risking Mom’s future care. I’ve seen this happen often with trusts that do not provide a Trust Protector (checks on the Trustee’s following trust instructions & the issuance of annual financial reports by Trustees). I wouldn’t go the route of challenging Trustee to recoup $, I’d file for Guardianship which is, normally, a lower Court who will focus on future care of Mom with $ needed for Mom’s care (recouping $, if necessary, should be easier as Guardian.). I hope it works out quickly; there’s terrible stress in situations such as this. The wording of the Trust is important, though it should not be the prime focus in Guardianship determination - only in that Mom will be left with no resources to maintain her needs. Check for a Guardianship attorney in the area.

I presumed Dad was now incapacitated & legally unable to change the executor. If Dad is competent to sign an addendum to his Will, changing executors, have Dad go to an attorney to change the named Executor - best if he went to the original attorney if there is a chance of a contest by sister).
As long as Dad is competent, he can change anything/everything. Help Dad get to an attorney.