My father is constantly bringing up what he feels were wrongs committed against him many years ago. He wont let go of the past. Recently, he threatened to hit my sister who is his main caregiver. He is always criticizing everyone and nobody wants to be around him. my mother is still living but will not stand up to him or critisize him in any way.
Personally, I'd probably go with him to a doctor's appointment and ask the doctor for something to mellow him out. That, at least, would be a start.
Your parents have been doing this dance for a long time. It's your sister who has to put on a different record. Is she living with them, or they with her? What happens if she moves out, in the first scenario? Is mom capable of caring for him and herself ?
Yes, someone needs to let his doctor know what's going on.
Here's a link on rumination.
https://en.wikipedia.org/wiki/Rumination_(psychology)