Oh... what rough stuff!! I feel for your sister, how hard to be going through a separation and now feeling like her son is getting out of control. How hard for your nephew. Being so little and having all this change with his parents and not knowing how to express his frustration, resorting to being mean and hitting. Tell your sister she is in my thoughts and I know how hard it can be. I am sure she is doing the best she can and must have a lot of self-doubt now. My advice would be for her to take a deep breathe and take a look at the big picture. Did this just start, or has it been leading up to this? Can she find areas that seem to be where he acts up the most? Being separate from the other parent is really hard because that much needed consistancey that everyone is mentioning is hard to keep. You really can't do anything when your child is going with someone else and what they do.
Then I would suggest she deal with every instance the same. I personally found it is different with each child, I have three. I would immediately get down on her child's level, you know eye level. It is hard at this age to really reason with a 2 1/2 year old because of where they are developmentally, but she should very sternly tell him, no, we do not hit, it hurts. Or no, we do not scream, it hurts our ears. I think they need to hear simple explanations as to why they are being told no. She can also say that he is making her sad. Removing him from the situation that he is in, that he is acting out on may help too. I know that when my son who just turned three starts to act out, say he throws a toy at his cousin, I go and lead him by the hand and say we cannot play with that because we are throwing and redirect him to a quiet activity. If he refuses and throws a fit, I let him. I distance myself from him and say I understand that you are mad, but we do not throw toys. After a couple minutes I will go to him and tell him he needs to say he is sorry and then we can pick something else to do.
When her son is behaving and doing what he should, she needs to praise that. Saying things like when you say please, that makes me happy, really seems to help. Children this age need to see a reason to do things. They are at that point where they have mastered all these things, so to be told no is very frustrating. Emphasize the good behavior.
Also, with all the transition that he is going through she really should make a conscious choice to be sure that each day she is spending quality time with him. He sounds like he is really needing attention. I know that is hard, single mom, working, but even if it is 15 minutes in the morning and 1 hour at night, find activities that he is craving and excels at. Emphasize how important time with him is. Make him feel very wanted and loved.
I know it's hard, and I hope this helps. If she needs any help along the way, tell her to feel free to e-mail me. I also have a 3 year old, so if she is at all interested in setting up some playdates, that would be great, if she lives int he twin cities.
I know that you will be there too auntie. I know when you are a mom, newly single and dealing with your child that is making you feel like your doing something wrong, it helps having a caring friend/sister to get you through.
J.