How does one stay calm when someone is abusing an elderly family member?

I am very sorry for your loss. There is much injustice in this world. I honestly don’t know how you can prevent these situations from occurring. I feel it is up to an individual person’s family to investigate these matters. Outsiders don’t have any rights to any personal or private information of others. If you suspect something is wrong then all you can do is call APS and ask them to investigate the matter.

I've found that the battle for calm in a stressful situation is won by remembering rage/wrath prevents clear thinking, something desperately needed. Deep breaths, exercise, prayers for God's excelling peace helped me to find resources. Some came too late to help my LO but I can help others with what I now know.

Your local Adult Protective Services office may've been a better resource than State level agencies or NCEA, which is more an education/resource provider.

The latter does point to ways to work within the community to assist/protect the elderly and disabled. You couldn't help Dad, but perhaps you can help someone else's LO by knowing what local resources are available. There can be a certain sense of peace in that.

A thousand hugs as your grieve your loss, cherish memories, and take care of yourself. In time you'll find you're able to put aside the anger and resentment of the past few years.

Did your Dad suffer from a Dementia? If so, he could not marry because he was not able to sign a legal document.

Unless our parent is under our roof, hard to protect them.

We are hearing only your side in this story, which is often the case here. Assuming you were correct that your father was abused by a woman he chose to marry I am also assuming that, at 73, he was judged incompetent. In that case it would be up to family to intervene at the time, to get guardianship of a father who is no longer mentally competent in his own decisions. Adult Protective would have been the agency to consult with evidence of being incompetent and therefore vulnerable, with evidence of financial fraud provided.
Sadly, it is now too late. You have taken what steps you are able to. Your father is gone. You say that you have wonderful memories of your time with him. Please try to concentrate on that. Anger kills us from within.
I wish you healing and peace. Perhaps only time helps in situations such as yours. I hope you are able to move forward knowing you have done what you are able to do.

I don’t have good answers for your attempts to rectify what occurred. It’s heartbreaking to see someone we love treated poorly. If your dad was of sound mind when he married, there’s likely nothing to be done. And probably not even if he wasn’t of sound mind. I’m sorry for your loss and wish you peace and comfort