Any advice? My nephew is so misbehaved! Every time I say something to my sister she says I am picking on him!?

My nephew is 11 yrs old, I see him every weekend at my parent's house for dinner. His behavior is out of control. This last Sunday he was taking up two seats on the couch and I asked him to move over, he refused to move over, I finally squeezed in beside him and proceeded to eat. He then TOLD me(not asked me which would have been ok) to shut my mouth when I chew. In all fairness I might have taken a bite or two with my mouth open because I am about 6 months pregnant and was ravenous at the time. But even if I did, I am an adult and he is a child and I don't think he should be talking to me that way. He then proceeded to grab a handful of his spaghetti and smear it on the table, at which point I told him he would be cleaning it up and he said "No I won't have to, just watch" Then my sister said she was more concerned about him eating than the mess and refused to make him clean it up. He acts like this 90% of the time, he swears and she does nothing, he hits and she does nothing. If anyone gets sick of it enough to try to discipline him she steps in and stops it, she says we all pick on him. I love my nephew and I really do enjoy seeing him when he is not being disrespectful, but those times have become few and far between. I am afraid that one day soon I will really purely dislike him because he has no boundaries. Even my parent's have begun to avoid him because my sister gets so mad if you try to say anything to him. I am so angry with her because I feel like she will ruin him and he will grow up to be spoiled and rotten! He is 11 and acts like a three yr old!