Y Y to getting hearing checked - good to rule that one out.
How to Talk So Kids Will Listen (etc) is a GREAT book.
But it's also about (as PPs have said) making sure your message is clearly understood by the child. EG "don't do that" doesn't make sense to a kid - they don't realize that the fun drum they're banging goes RIGHT THROUGH YOUR HEAD!!!
Try and use positive language where you can... there's some evidence that using a sentence with "don't" in it, kids hear as "do". EG "don't walk on the grass". Two seconds later "why are you walking on the grass" "because you told me to!"
Here's what I mean....
Don't think about apples. Don't picture apples in your mind.
Don't think about the shiny apple the wicked stepmother gives to Snow White?
Don't think about the crunch when you bite into an apple, or how it tastes in your mouth.
See what I mean?
Try and find ways of framing what your saying so you name the behaviour you want rather than the behaviour you don't. (not "don't stand in the mud" more "please walk on the path")
And give praise, praise, praise when you get it. Always notice and thank.
The other thing I recommend is giving 2 CLEAR choices and a relevant consequence. EG You can keep being mean to your DBro and not have a story/pudding/extra telly or you can be kind to your DBro and we can play a nice game together. I'd like to play a fun game with you, but I can't do that if your DBro is crying because you've niggled him.
Hope some of this is useful!
Ignore where you can.