Okay, I know that this is just SO difficult and maybe painful.
But, I will offer some simple, sage, advice, that might help.
Remember, you can not be responsible for anyone else's behaviors, actions, attitudes.
Only your own.
I would not 'beg'.
I would not exclude anyone.
I would not make excuses....
I think you should make sure to have nice plans made, and set, well in advance.
Put the invitation and information out there...
And how everyone else happens to respond is really up to them!!!
Remember, if you 'beg', or 'ask for a truce', then to your brother the implication in his mind will be that he is being blamed for the need for a truce. People who are perpetually offended and 'falling on knives' will defiinitely do so, once again. To say, can we call a truce, or can you 'be nice for one day', will be heard as you are a jerk and can you not be a jerk. I can see that going over like a lead balloon.
Send the invitation out.. State clearly that "Dad would love to see everyone there. It would mean a lot to him..."
Do what YOU can do to make this event happen.
And just know that you have done everything possible, and you really have nothing to regret or feel guilty about.