But my son wants his cousins there, so hope all behave.
Same here. The only reason why I have any relationship to my sister at this point is because of her daughters. They are the priority, and if we all have to deal with my sister's crap so we can show them how much we love them, well, we'll have to deal with my sister's crap. It's a price I'll happily pay.
I don't know about you, but I did spend too much time and energy getting angry whenever my sister was at her charming best, because people would question my judgment. I found that it was best if I never said an ill word about her in public - it just made me angry and bitter, and life's too short for that. And in my case, after many years, people realized what I had been dealing with.
In regards to your parents, people are going to perceive what they want to perceive. I've learned the hard way that sometimes, especially when you ARE close to somebody, that you cannot be the messenger of truth and that role must be taken by someone else. And sometimes, even the truth isn't enough for them to open their eyes. My cousin physically assaulted his younger siblings and sent them to foster care, AND he sent the mother of his child to the hospital and his parents still stuck their collective heads in the sand for many, many years.
You have so much on your plate. I hope the day goes well.