Mean sister

How should I discipline my Princess not to hit her brother?  How should I be strict yet gentle on her? 

I know that Princess is an attention seeker and sometimes she may not know but doing it out of attention seeking.  But hitting on her younger brother is definitely a "No".

I have been told by baby sitter numbers of times that Princess will secretly hit her brother when she did not notice them.  And now my MIL also complained to me that Princess has been hitting Baby Jay not once but twice in hometown.  And one time she purposedly pull the pillow of Baby Jay from his head causing him to cry.  When asked by my MIL, Princess lied that she was helping to adjust Baby Jay's pillow for him, not pulling away. 
The latest incident was few days ago.  It happened in canteen.  Princess told Baby Jay to do something and when Baby Jay did want to, Princess slapped him on the face.  And imagine poor Baby Jay, cried non stop.  My MIL said with the slap mark on his face, Princess still lied to her that she did not do it.  Sigh!

So you see, I was in dillemma now.  One is that my Princess knows how to tell a lie. Secondly she hits her brother when brother does not listen to her.

Guess patience and abstinence will work out in her case.