Eek, there's a lot of things here I'd find rude.
I find it rude that they were talking almost completely in Polish and no one thought of including you in your own house.
I find it rude that your boyfriend isn't that considerate to you in any of these situations, because subtle signals and attempts from him to translate or include you might resolve this.
I find it rude that you didn't wait until the house guests had left before having an argument with your boyfriend about it.
I find it rude that she butted in on an argument between you and your boyfriend when she was a guest in your home.
All that said, I think you should try and go to the wedding anyway. If anyone was guilty of being more rude, it was her. She might be defending her brother but totally inappropriate to get involved in a domestic when you're a guest in someone's home. I'd never do that, no matter what was being said.
All that said, you have to allow for cultural difference though, because us Brits are massively over polite and have all these rules about house guests and manners that I don't think apply anywhere else in the world quite as much. Some countries believe in being so much more open which might be the case here. For us, we are almost pathologically polite!
You don't have to feel like people will think badly of you though, it's a pretty small thing and she is likely embarrassed about it too but you both love her boyfriend so going just allows you to move onto the next chapter and put a silly argument behind you. It sounds like they are close and it'd be a shame to put a little thing like that between family harmony.
Just make it clear to your BF that if he wants you to feel part of things with his family, he has to massively step up to the mark in making sure you don't feel uncomfortable around them, and excluding you completely is plain inconsiderate. Fine for them to have some chats in Polish, but not to completely ignore you like that.