Wow, R., what a nightmare. Listen - I think you are absolutely in the right to discipline your nephew, especially when it involves protecting you and your family, your safety, and your sanity. So I would do even more of that, just try always to be very loving about it, and calm, and mix it with lots of affirmations of the nephew whenever he does anything that isn't misbehaving. But when he screams, perhaps give him a time-out in another room until he can sit at the table without screaming. If you are hosting gatherings at your home, you should be able to have rules for how things go in your own home. They can make their own rules for their home, but in your home there is no screaming at the table. Maybe try to phrase it that way. And then you can always have certain foods - desserts or something - that are for all the people who don't scream during supper. and if your in-laws protest, you can tell them they're free to parent however they like, but you're going to have a certain set of rules hold sway for your family as well - and in your home, people don't grab each other's things, scream, or hit. I would try to make it as much about YOU and your family, as possible, rather than about THEM, to avoid friction. And like I said, affirm the boy if he says or does anything nice - "I love how fun you are! You are sitting there so nicely right now! I can see you love your cousins so much!" Whatever you can think of.
Good luck! That's a real tough one!!
T.