Aggressive seven year old son

So my seven year old son has started getting aggressive lately. Mostly minor things but two things recently have gotten my attention. He has always been very mild mannered but about a year ago my sister's family and I joined households and she has three rough and tumble boys, I thought that it would be good for him and help him to come out of his shell but now he acts crazy. I'm not sure if it their influence or just the growing up proccess. He has started hitting his cousins and he actually shook his fist in my face the other day! The scariest part is that he has a 15 month old brother whom he tried to smoother recently. I am afraid to leave the two of them in the same room alone. It's just so sad. We used to have a great, sweet relationship but now he scares me. I've always prided myself on being a good mom with obedient children but not anymore. Help! Is there anyone else out there having this issue?