‘It was a mistake’: My father set up a revocable trust, leaving everything to my stepmother. She’s cutting me out completely. What can I do?

Dear Quentin,

My late father was under the impression that he and my stepmother had an irrevocable trust set up. He was wrong. He passed away in September and now we have learned it is actually a revocable trust. It was a mistake. My stepmom is letting her son handle things for her, and has talked her into changing the trust so that he will get everything when she passes, so he can help take care of her sister who has mental-health issues. 

This…