9 year old ds1 and 5 y/o ds2 fighting

[Jnice]

I tell my DS I have no tolerance for hitting etc. I they hit/kick etc it is an immediate 2 day grounding, which includes no screens. This evening I saw ds1 punch ds2 in the arm. Not very hard as ds2 didn't cry out. I told ds1 he is grounded and now he's crying out how unfair it is for all other times ds2 has hit him without consequence. I cannot always witness everything!

Bottom line is that I expect ds1 to lead by example and not hit someone almost half his age. I don't care what made him do it. Is this unreasonable? How do you handle this kind of situation without becoming a constant influence and negotiator for them?