S.Z.
I discovered, when my children did this, that it was a way of grabbing Mommy's attention. Your son probably resents the time and effort you spend on his sister, even if he loves her (and even if he's unaware that he feels resentful.) He may be thinking that if you are "angry" at her, or if she is "bad," that you will love him more. It sounds ridiculous to an adult, but makes perfect sense to a child. He sees your disciplining efforts as the perfect moment to show you that she does bad things; in his mind, you will admire and love him for this. When it upsets you, he doesn't understand why. I finally discovered that I had to send one child out of the room, immediately and unemotionally, every time they interfered with what I was saying to their younger sibling. This can be hard, especially at the table, but he should catch on very quickly. Even if it's at the table, when he starts egging her on, ask him to leave the room. Say something like, "I need to speak to Ella right now. Please go wait for me in the other room." Then call him back or go to him when you are done speaking to your daughter. He'll test out the new program by making you send him away every minute or two at first, and he'll probably complain, but then he'll probably decide that it's in his own best interest to remain quiet, and in the room, while you discipline your daughter. When he finally does that, tell him that you really appreciate it. He has to decide that it's better for him to do the right thing, "better" in his mind being that he gets your complete attention.