How much "NO" is too much???

I found that I've been saying a lot of "NO" these past couple of months. Ever since Colin seemed to understand more of what I'm saying and ever since he learned how to annoy his sister. I saw a couple of times that when Colin wanted to play with Ava and her toys, she would scream at him and pushed him. The screaming I can tolerate, but definitely not the pushing, so I said to Ava, "Ava, no pushing.. you can't push didi coz he's still a baby... he just wants to play with you.. ok??" And Ava will answer, " NO.. NO DIDI!!! Go away!!" And the screaming and pushing would continue until I separate them. But on the other hand, Colin would cry because sometimes he would hit his head falling backward from the pushing and sometimes because he just wanted to take the toys. So I would again say "NO Colin.. Don't take your sister's toys. She's playing with them right now.. you take turn .. ok??" Colin's answer was," wa..wa..wa..wa............" . .. (*sigh*)

So, how much "NO" is too much??? I tried to NOT only say NO to Ava because I'm afraid that she will think I favor Colin more and she will probably try to hit or push him more (esp when I'm not looking)... but I can't help myself saying other words besides NO because it just came out so naturally.. (*another sigh*)

I remember a story my mom told me... A long time ago, there's this little boy who was so spoiled. His mom never said NO to him and had always allowed him to do whatever he wanted. His mom also never thought him right and wrong, so the little boy was always taking other people's things and never knew that it was wrong. This little boy grew up to be a thief. He started from small crimes and gradually moved up to bigger and bigger crimes. He finally got caught was going to be punished. In the prison, his mom visited him and cried when she saw him. At this time, his son said to her, "This is all your fault. Why didn't you ever told me what I did was wrong?? why didn't you teach me??? You should have told me what's right and what's wrong!!!!"..
When my mom told me this story, she also told me. Sometimes, parents are to blame when their kids did something wrong because if the parents didn't teach them from when they are young, how will they understand?? Parents job are teaching their kids everytime they see that their kids need learning. Right now, I'm teaching you how to be a good mother and if you have any doubts or questions, you know where to turn to..