My elderly Father has gotten very mean and won't let us get him help. What can we do?

IMO threatening physical harm is the point where action is needed. Does his doctor know this? He needs med adjusted or evaluation to see if something else is going on too? Is this a decline in mental status, infection, dementia, or something else?

Here's a link on rumination.
https://en.wikipedia.org/wiki/Rumination_(psychology)

then your sister is not being forceful enough. You go with her next visit...or make a sooner appointment. Don't leave without medication or a referral to another doctor who is more responsive. Doctors need to be pushed into a corner at times. Between you and sis, I suspect you'll have a different outcome.

my sister is almost at that point. she has gone to his last few doctor appointments and has talked to the doctor alone, but he has done nothing to help the situation. the doctor more or less tells her when he sees that they need help he will act. the doctor doesn't seem to accept what my sister has been telling him, and my parents put on a good front for the doctor.

It's called rumination. There are meds for that, as Maggie mentions.

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.

Until your sister, his main caregiver, has had enough? Nothing is going to change. Until she decides to enlist outside help regardless of dad's protests? Nothing is going to change. It's all on her.

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.