Siblings hitting each other

Mine used to do this, the worst period was when they were under 10 although it really only stopped totally by the time they were both in secondary school.

My advice would be to make sure you punish both children, although I would never condone the hitting, often that was a last straw reaction to the situation that was created by the the other one.

After breakfast today, keep them at the kitchen table and tell them to listen without interrupting.
" Today is a day of new beginnings. From now on, there will be no hurting in any form, kick, punch, pinch, . I can't force you to get along and love each other, but I can force you not to hit each other. Every time you do so, things you love and cherish will disappear. You won't know what until it is gone, could be your favourite toy, dress, or even your bicycle. There will ben no warning. You hit, something is gone.
And from now on, the things you like to do, like watching screens or having a biscuit, will need to be earned by being kind and respectful to everyone in this house. "