Ahh...J....I feel your pain. I have two boys, 18 months apart. They are now 5 and 7 (nearly 8), but this situation is always an issue when you have siblings. It just evolves as they grow up.
One of the problems that you have here is that your 2 year old is still in what I affectionately refer to as the "puppy stage." It's the stage where no matter what you say to your child, you feel like you're training a puppy. You know: "Get down! Don't eat that! Shhh! No 'barking!' Lie down! Go to sleep! Do you need to go potty/poopy? Stop chewing on that! Take that out of your mouth!...etc, etc, et al."
I came to a realization when my boys were little..."training" them to learn right from wrong is JUST like training a puppy! Think of all the things we teach them...and then think of how we train puppies...it's a weird similarity!
The reason is this: Children and puppies are alike in that they do not have the ability to reason...they have no critical thinking skills. So all that explaining you are doing..."Oh, no, honey...you can't do that because it hurts your brother" or "it makes me sad when you do that" or really any other way you try to explain it is going to go in one ear and out the other...because his reasoning skills aren't developed enough to understand your explainations quite yet.
So what to do? Well, you have to break it down to a level that he can understand. Take the verbal reasoning out of the equation...because it's your job to TEACH him how to reason. You do that with consistency and consequences. For a puppy, it's pretty easy...and I applied it to my human child in the best way I could: Here's an example...
Say your puppy is doing something he shouldn't. He's got one of your brand-new patent leather 3 inch heels and he's going to town on it. Nom, nom, nom! ;o) You've caught him in the act...but he doesn't really know that what he's doing is unacceptable because you haven't taught him yet! So you have to teach him. You go over to him, take the shoe out of his mouth and take him by the collar, crouching down to his level...and you say firmly (use as deep a voice as you can make...it's proven that a deeper voice receives more attention in both puppies AND children) "NO! NO! That's NAUGHTY! We do NOT eat shoes!" Then you take him completely out of the area...away from all attention, and put him in "time out." For a puppy...it would be in his crate in a quiet room away from everyone. **In order for him to understand that he cannot chew on your shoes (or whatever), you have to also attempt to redirect him to things he CAN chew and have them available to him...so he can make choices for himself when he finally learns his lesson (which may take a few times).
So...it's pretty obvious to see how this applies to your 2 year old. And when it happens, it needs to be quick and concise. He pushes brother...you immediately "catch him" in the behavior (there shouldn't be any break in between when he does it and when you take action or he won't understand what he did). I would not pick up your little one...big bro needs to see him cry. Comfort him after you take action with big bro. Right when it happens, crouch down to his level close to him, take him by the shoulders (just so you have his full attention) and say "NO! NO! We do NOT push/hurt our family!" Then take him by the hand (or pick him up if he's unwilling) straight to his crib or another place where he can be safely isolated and not have any attention paid to him (positive OR negative). Leave him there for 5 minutes, or when he stops crying...whichever comes LAST. Don't peek in on hom or say anything to him...he should be getting no attention at all. When you take him out, do NOT explain anything...just consistently respond to his negative behavior that way EVERY time. It won't be long until he associates his behavior with a consequence that he isn't fond of.
Never threaten. Just do it. I can't say that enough.
I know it's tempting to try to explain what you are doing...why you put him into "time out." But at this age, all you'll be doing is taking the focus on the reason you put him there...because he won't yet understand your explanations. Save those for when he's 4 or 5.
I've found this technique to be good for almost any negative behavior...it might need to be modified, but it works.
Good luck with your "puppy!!"
:o)
C.