Almost 3 Year Old Nephew Has Anger/bullying Proglems

Kids aren't bullies at three. I know because our son was like that at three and we were so worried about it, but the doctors said it wasn't possible to be a bully just yet. We ended up seeing a child psychologist and moved along in the process to learn our son has ADHD. At three without treatment, he couldn't control his body at all. No impulse control like normal three year olds have to some degree.

If you're worried, you might mention that you really didn't want to say anything, but you're worried that so-and-so has problems with impulse control and was scaring some other kids and bruised your son. Don't try to diagnose or criticize, just offer your feedback, noting that as a parent you thought he/she would want to know. I know from experience how hard it is to be the parent of the child with impulse control problems, but also know that it's incredibly helpful to know about all instances.

Good luck!

He's mirroring something he's seen. Now that you are aware of his "issues" just do what you have to do to protect your child. It's hard to tell another mom what to do. You can tell that just by reading posts here, boy if you step on someones toes they will sure let you know.
So, you cant control that child, you can only control and react to what you know. Since it's family and you have to be around them often you may want to bring it to the moms attention in a concerned but nice way.... Chances are she is freaking out about it herself.
ADD ON: Could it be sugar from the halloween candy? Some kids cannot handle the sugar rush and it makes them aggressive.

Are your nephew's parents doing anything about it? Do they attempt to discipline and correct him when he exhibits this behavior? It might be normal, or it might be on the more aggressive side of normal, but that does not mean that it is acceptable or just let it slide, figure its no big deal, and that he'll grow out of it. You could try saying something to his parents in such a way that does not put them on the defense (which may happen anyway, because some people are just really touchy and defensive) but does bring it to their attention. As in,"I noticed some bruises on my son's arm from where your son grabbed him - he seems a little more aggressive than some other kids I've seen, is everything okay?" Emphasize that you are more concerned than anything, not trying to judge or tell them how to parent - but I can't promise you they won't react in a less-than-positive manner. You know their personalities better than me. There could be something going on at home to account for their son's behavior, or it could be that they just are not correcting him and expecting better behavior from him.