Basic chart that is age appropriate and that they can understand.
It's not the idea to reward for not doing something... it's positive reinforcement of the behaviours you want to see more of.
For example (with the info you've given):
Playing well with Bro
Being nice to DBro
Saying please and thank you
Basically think up the behaviours he does that you don't like and turn them on their head - what do you want him to do instead?
I'd keep it to five or so categories. Get him to "help" you make the chart (both by thinking up the categories together and also by allowing him to decorate it afterwards a little - maybe he could help write his name, or do a squiggle or stars down the side). He's feel far more invested in it than if you present a generic bought one with too much going on (either prescribed behaviours or too much space for categories). A simple piece of paper and drawn up with felt tips is just as effective.
Present as an exciting new thing where if he does nice/good things, (if he understands "behaviour" then use that word of course) he'll get a reward after a certain amount of ticks! and what nice/good things could he do? Should you have a think together?... like... Oh I know DS, how about playing well with DBro? etc. Basically suggest things you want him to do like it's just occurred to you and ask him if that would be a nice/good thing.
He can then see the ticks building up on the chart. I'd pick something small but affordable such as a kids magazine or a new box of crayons or some such small item which you know he'd like. (You can also get these in advance and hide away so they are ready). Try and make it so it takes him maybe 5 days to earn it (any more and they lose interest, any less and it'll be too easy - of course it depends on the child, their attention span etc. Sometimes a really small reward per day might be appropriate, like an extra story, or coloured bubble bath or use of bath crayons or whatever).
Never threaten to take away previously earned ticks if bad bevhaiour arises. Whenever he is veering into undesirable behaviour territory, remind him that he gets a tick on his chart for being nice to DBro and is taking DBro's toy away nice or not? Get him to think about his behaviour and self-modify where possible. It's a lot nicer than punishing or shouting
Obviously, if he did something that did require a telling off then you should still do that but ultimately, where possible you want to set a new pattern of behaviour, it won't happen overnight but gradually it will form new habits. Catch him doing something he can get a tick for on his chart even if he's just sat there not annoying Bro - say "DS, I see you are being nice to DBro! Well done! I think you deserve a tick!" etc. Be ready to hand out ticks early on so he gets the drift, it doesn't all have to be about the big moment of heroic displays of sharing and self-control. He will learn that sat either letting DBro alone and/or being nice gets ticks (and positive attention).