Remove Them From the Situation
For me, quite literally walking away when my toddler hits his sibling is often enough to diffuse the situation. But if he can't get control of his emotions and doesn't stop hitting, I will sometimes, without saying anything, calmly remove him from the situation and put him in his room for a break. I don't know if Ralphie would recommend this, but it's my way of following her other advice to "ignore the behavior, not the child" and seems to be effective in more escalated situations.
If both he and his brother don't stop even after I've ignored them or if they are continuing to figuratively poke at each other, I will try to change the situation. I still ignore the hitting and fighting, but I'll go over and say, "Boys, it's time to play outside!" or prompt them to move onto another activity. Often, simply redirecting them gets them to snap out of it.