Is it okay for me to/how do I get out of the upcoming family holiday?

Last summer dad dragged me (19 at the time, 20 now) and my 2 younger siblings (sister - 16 turning 17 in a couple days and brother - 12 years old) on a holiday in Devon. We all hated it, and we did our best to hide that we hated it, but he could tell. Mainly because we'd done the exact same thing every year since we were born (and dad has done it for longer than that). He's not a great dad, tbh. He yells a lot and he always clashes with my sister. They're too similar. They can't spend a day together without a fight, let alone a week. When I go on holiday with them I involuntary become a buffer between them. And when I say buffer I don't mean that I can diffuse arguments, but when I'm there they both choose to mock/insult me rather than pick at each other. They call me stupid, make fun of my appearance, and mock me because me and my boyfriend are long distance (they joke that he isn't real) when the reality is that we see each other all the time - I just don't want him to meet them.
I'm sick of it. It's not healthy for anyone involved. I don't want to be stuck in the middle of the two of them for another week.
However, I feel guilty. If it weren't for my brother, I wouldn't hesitate to bail. But my brother was there for mum and dad's divorce, he listened to all the fighting, same as me and her, and he doesn't do too well with arguments. Neither do I, tbh, but I'm an adult and I can handle it better than he can. But I already go with them every other weekend to see dad and every single time I get both him and my sister attacking me from both sides about literally everything.
I'm 20 now. I think I could maybe get out of the summer holiday this year. My questions are: 1) is it justifiable to leave the 12 and 17 year old to deal with dad and 2) how could I get out of it? Dad won't accept that I just don't want to go. I'd need an excuse I could start setting up now and give him in the next couple months before he makes any plans.

Tell him you're an adult and you have your own life so you can't make it to the family holiday. Your siblings can't always have you around to make things easier.

This is a tricky one. I don't think there's any single best solution to this. There's scope for a range of different approaches to be used all of which would be equally valid.

This is one of those times when it's probably best to trust your instincts on this.
It's also one of those when different people would have different styles in handling this.

This will be an exercise in handling conflict.

One valid solution would be to tell your dad that you care very much for him, your brother and sister. But that in 2019 you don't intend coming on the summer holiday with them, because you didn't enjoy all the stick you got in 2018. Offer to let your brother and sister stay with you for the holiday period and for your dad to go on holiday as a free man without the three of you.

Another valid solution would be to say that you will only go on holiday in 2019 with them on the agreement that there will be a zero insult policy. Any insults and you will catch a taxi and train or coach home early. If you proceed with that, you could set a 3 insult limit in your head. After the first insult say "We had a no insult limit on this holiday, please do not do that again". After the 2nd insult say "I've already warned you about the insults, this is your 2nd and final warning." After the 3rd insult, phone the taxi company and get out of there. Taking brother and sister too if they've had enough.

There are other perfectly good and reasonable approaches that other posters could add to this thread... Select two that you think will work best for you and your family, a main one and a back-up plan.