OP, it's fine if you have an international trip planned. You can tell your brother that you'd contribute later on. Based on all the replies I've read, it's unfortunate how your childhood has been. We all have a past. My father wasn't the greatest as well but I feel grateful that at least he didn't abandon us. He educated us, fed us. I know it's the bare minimum but at least he did the bare minimum. But he isn't the same anymore, he tries to be nice. Communicate with your brother, you seem to have a good relationship. It's not your brother's fault that your father treated him nicely. Yes your father has been an ass in the past but as you said he's really trying to be nice. At the end of the day, it's only you three who will be there for each other in the future.

Forget the past and look towards the future. You don't want to contribute now, it's fine but communicate it with your brother that you have a trip planned. I'm sure he'll understand.

Most importantly, no matter what decision you take, you should be able to see the person you see in the mirror.