Not OP, but depends entirely on age and how you parent, everyone is different and every child is different.
In our household, it's always be a firm line. For very small, it's a firm 'no' and removing the hand, and if they continued, it meant putting them down/stopping play. Once ours were older, removing them from the situation tended to work better. For example, if they are playing with their sibling and hit, it's a "no thank you, we aren't playing anymore/I don't want to play if you are going to hit" without discussion and the offending child is removed (told to go to their room usually). Reaffirming that we don't hit, it isn't kind, it isn't polite etc.
Social pressure (wanting to be part of the gang and not wanting to be left out) tends to be a good reminder. If you react with anger (which is hard not to do sometimes!), it is still attention and it becomes a game. A calm, no nonsense approach and letting the child figure out for themselves that hitting will get them absolutely nowhere works better for us. After they are calm and have had time out, I make a point of asking them if they are okay/why they hit and coming to a point of them apologising to their sibling. Sometimes it's from being overwhelmed, sometimes it's from frustration. Trying to understand, to model the behaviour you want and offering ways to deal with the emotion (we do Daniel Tiger's 'when you feel so mad that you want to roar, take a big breath and count to four"), and praise, praise, praise for positive behaviours.
And to keep this on topic for the SJ's, not encouraging or laughing at sibling fighting, especially when it's physical is a very simple place to start. Sadly J + A don't care, and the kids now do it to earn their parent's attention/affection.
Just look at her making a complete ass of herself 
