I see Beatrice has a lot of 'backup'!!
And WOW, it's great that you guys stood up for your sisters etc!!
It helps to know what you'd like from him, specific behavior. If you just expect remorse or apology hmm.. (not so sure about that, my dad wasn't remorseful when we got into a confrontation about the past, nobody really likes the blame, it's dropped like a 'hot potato') Maybe it's better to focus on the present and what could be done to make each other's life easier etc?
There's a book 'Dealing with Difficult People' by Rick Kirschman and Rick Brinkmann (the longer version) - see if your library has it? It was helpful to deal with Dad in the past too.. The 'anti-tank' strategies... It's a really good book, so I recommend it.. (it's for general stuff though, not specifically for past trauma or any abuse issues or such...)
If you think the person might get violent or aggressive, there are probably things you could do too.. Like maybe having police telephone number in your cell phone to speed dial if necessary, or people to go with you, like other helpful relatives that Grandpa might listen to or respect? or maybe recording the conversation if neccessary etc.
A friend who was a waitress also said to a yelling guy who threatened her, 'I hope you or your Dad have a lot of money.' - 'Why?' 'Cause I'm gonna sue you if you even put one hand on me.' or something like that.. Not sure if saying something like this would be a good idea, maybe it's good to have a few 'ways out' if things would turn uncomfortable.. (Or people to call you/check up on you/etc?)
Also know maybe he's yelling cause he's deaf or half-deaf (bad of hearing). Some people just yell louder cause they think other people can't hear them either. And sometimes if you yell back they're insulted (that's a hearing problem, it has nothing to do with you personally!! It's good to remember it!!).
Hope things go well!! My Grandpas were really nice and I hope you find a way to talk with your Grandpa too!!