I'm so irritated with my nephew for being such a spoiled brat, and my sister for condoning such a disrespectful attitude.
What did he do?
He is constantly calling me names, punching me, and snapping at me when I give him orders -- something I do not appreciate from anyone else. Now his mother butts in and pampers him, chiding me for berating him. Now my mom joins him, and instead of siding with me, she sides with them, which is so really irritating.
Do your sister and nephew live with you, or are they just occasional visitors?
This is just a suggestion, but maybe you should try modeling to your nephew how you want him to behave by speaking kindly and calmly to him and paying attention to him when he behaves appropriately, then walking away from him and completely ignoring him when he behaves inappropriately instead of giving him any negative attention for it by berating him. Tell his mother (or yours, if his mother isn't there or refuses to deal with it when you do tell her) when he hits you and have her deal with it.
I tried to find "How to deal with an Obnoxious Nephew," but all I could find was this video, which seems more geared to parents:
http://www.livestrong.com/video/1730-ignore-obnoxious-child/