My older child used to pick on the younger relentlessly, delibarately tease him. My younger child had ASD, and the older one had Dyspraxia. Looking back my advice is to set very firm boundaries. But that isn't the same as consequences or punishments for hurting the younger. That didn't work at all.
Basically the older one was attention seeking, and had his own difficulties and frustrations which he found a quick fix in attacking the younger. It seemed an uncontrollable and easy way to get the attention he craved or release his own feelings of low self esteem or failure socially (my ds1 didn't have any friends and the younger did)
What worked was to be incredibly attentive to the older one, try and work out what was bothering him, reassure him that he had lots of qualities, generally build his confidence, WHILST AT THE SAME TIME reminding him that the younger one was under my protection and I WOULD NOT ALLOW HIM TO BE HARMED OR INSULTED.
But how do you stop this you may ask? I would separate them or do separate things with them before the tension built up. Or if they did things together they wouldn't be competitive things or things involving choices or sharing juice for example or toys. So for example we never usually let them watch telly together, as ds1 would use that as an opportunity to get at ds2 with words. But sliding on slide or splashing in the water or silly play worked better as a bonding exercise. Lots of lots of time spent giving ds1 my full attention as so much was going to the second (who I adored - he was a very lovable character - ds1 less so - the SN in our situation was not the troublemaker but the victim)
I also found reading Non Violent Resistance very helpful later in reinforcing a lot of these ideas that I had in a hit or miss way been implementing. The idea that you can and should intervene whilst making conciliatory gestures and "reinforcing parental presence"
It is hard to explain it, I think it comes from looking at things from the beginning rather than just responding to situations, which of course is what you start off doing, why are you fighting, why are you being mean, don't hurt your brother? None of which imo, work, and the child hears nothing when you say this. Whereas staying with them and bolstering their relationship through "building blocks", perhaps a game like pairs, or squidging something, jumping on cushions. Anything really that builds self esteem and reduces frustration in the hitter and attacker, but that isn't the same as letting them get away with violence or aggression.
Consequencesof the punishment type tend not to mean much, and reinforce that they are the one with the problem, and makes the child feel worse and even more out of synch so to speak, which means the aggression can then escalate.
Sibling Rivalry by Faber and Mazlish is a useful book too. All this stuff just sounds like a chocolate teapot with what you are experiencing but you sometimes do have to start at the beginning. HTH. Sons are getting on well now as teens, but it was awful when they were 8 and 6