The title of this piece is what I often hear neighbours say to their children. It seems like a clear instruction; the noticed, obviously unwanted behaviour should stop. Is it a clear instruction however, when you have to repeat it until only a raised voice is making the difference?
In all fairness, from our perspective, children should understand what you mean when you say 'stop that' after just hitting their sibling. And we don't always have the time to have a lengthy conversation about what happened, who started and all that.
Let's try and be a few steps ahead of our children then. Do take the effort to specifically point out the unwanted behaviour. So, instead of saying 'stop it' or 'don't do that', try saying what you see. 'Stop hitting you brother', is a specific order, which then should be followed by an alternative. The alternative depends on your surroundings. If you are driving in the car, you can offer other alternatives than if you are at the playground. In case of driving in the car, you could say 'we will sort this out when I have found a parking spot and, in the meantime, you will have to keep your hands to yourself'. In case of the playground, you could decide to talk about the why and how, but when it happens a lot you can choose to tell them to play in two different corners or areas. I would like to support you in finding a structural and durable solution when brothers and sisters quarrel a lot.
Children need specific directions and instructions. Especially when they show unwanted behaviour, they need an immediate alternative that is realistic. Because, if not hitting my brother, than what? When emotions run high, coming up with an alternative yourself is a big challenge.
What can you think of to prevent unwanted behaviour in situations you know can be a challenge?