Strong willed?

"Strong willed? Not my child." I used to say that when Kai was younger but now that she is older I am just at a loss as to what to do with her attitude. She has got to be the most infuriating child on the planet. Two things have occurred in our lives the last two years that she has gotten over. One being that we moved from Corsicana to Houston. She doesn't understand why we had to move. We try to explain all the reasons why it was the best move for our family but she doesn't want to hear that crap. All she knows is that we took her away from her home, school, and friends. The next BIG change that has happened is that she now has a living, breathing creature in Jakob vying for our affection and attention from her. For over 5 years, Kai was the center of our little universe but now she has to share and she does not like it one bit. How do you explain to your child that their younger sibling is here to stay and can not be returned like a shirt that you no longer want. YOU CAN'T!! So we are open to any and all suggestions as to how to handle this situation that has brewing in our home for awhile now.