I'm going to report possible child abuse by my brother
What do you think your brother is doing?
His son always has bruises on him so I think my brother is hitting him.
It's good that you're going to report possible child abuse committed by your brother. Your nephew might be in a really terrible situation, and by reporting it to the proper authorities then you might be helping him in a big way.
I'm just not sure if it is bruises from my brother and it might be from a fight at school.
Okay, well it sounds like you are not so certain that your brother is to blame.  If you really have your doubts, then you shouldn't be so quick to alert the authorities, but if you have evidence to suggest abuse, then my previous point remains.