G.♣.
Have you tried separating them? (rather than punishing the oldest, even if it is his fault) When my boys start to fight (and really, most siblings do this to some extent), I usually just end whatever activity they are doing and have them go to different parts of the house. My boys share a room, so one off them can go there and the other can be in the living room. Or they can both go outside. If they really can't get a long, I give them something specific to do. Sometimes it's a chore. Other times I suggest a book or legos or whatever.
I'm not at all suggesting it's easy or it doesn't get on my nerves, because it absolutely isn't easy and definitely does get on my nerves! But I've found more success when I stay calm and just shake things up a bit with separation or new activities. Sometimes they'll be back to playing together and laughing within minutes.