So you are saying the 9 year old WAS seriously hurt? Broken arm, concussion kind of hurt?
It is not ok for a 16 year old to not 'get in' with a 9 year old. Be irritated by them yes.
On the other hand 16 year olds sometimes do terrible things on the spur of the moment because they really do not think of the consequences. It could have been a quick opportunistic daft move or it could have been a spiteful, malicious assault.
Is your 16 year old jealous / resentful of the younger?
I would confront the 16 year old directly as others have said.
And, before his brother can get to him I would talk to the 9 year old, say you have seen that his brother tripped him and ask how it happened and see whether it is a pattern, whether he feels unable to talk about it, feels intimidated by his brother etc. Make it clear he can talk to you and be believed.
My approach with the 16 year old would be very firm: we behave like a family to each other or we lose family priveliges. We talk about things (mean this, if you say it), we use our words, we listen to each other.
I don't know about punishment as such, or if it actually helps his attitude to how he behaves towards his brother. I would be looking more at strategies to improve respect, behaviour, kindness, communication.