I have a sen child and I learned a lot about preventing problem behaviour very fast!
Firstly, is there anyone who can help you to separate out the kids a bit for the next few weeks? Sounds like you are all on top of each other. An hours babysitting etc here or there? Any activities you can sign up your oldest for? Swimming? Dance? Sports? Art? After school or weekends.
Have you considered sectioning off parts of the room with stair gates or play pens, e.g. his bedroom with a stair gate or stair gate on a room downstairs? Just to physically separate the kids a bit. I know this is a juggle as you physically have to keep an eye on them. But even if used for half an hour say putting oldest to bed earlier, with a toy he hasn’t played with yet
Consider an iPad for oldest. I know he’s young, however he is craving attention and stimulation. He is probably bored too. If you cannot be giving him a lot of attention then short periods with an iPad can help and make him feel he’s getting something they don’t. It can also be a very useful reward for good behaviour.
Separate the day into chunks of activity. Not loads of time in the house or loads of time having to do shopping either. Make it focused on what the kids need. Get home deliveries. Don’t do housework or minimal. Minimal dinners. You just haven’t the time or energy realistically.
Make sure at least part of the day is outside. Walks to a shop for a small treat. Walk to play park. Walk to soft play. Or if it’s too mad walking with all of them, drive to soft play or something active for that boy. He needs physical activity.
Divide up the day in the house. Music on dancing for half hour. Books out half hour. Big toys out for an hour. Dress up hour! Like schools and nurseries do, they do it as it keeps a lot of kids stimulated and not all on top of each other.
Then give that boy praise every single time he does good. Be specific. You were very kind to your sister, when you shared that toy, well done.
Escalating negatives for bad behaviour, a stern NO for not sharing etc, an immediate consequence for any aggression.
However the best way is to set it up so that this does not happen!