I'm going to push my sister after eating a small spoonful of pumpkin pie
Why would you push her over some pie?
I ate some of her pie and she is complaining about it too much.
She has a right to complain to you about you eating her pie without permission. Next time just ask her if you can have some, violence is not the answer.
Tell her to stop complaining about it so much then, I only ate a little.
The amount you ate is not the problem, its that you did it without permission. You would not like it if you were enjoying your food and someone came and ate some without asking you. Just apologize to her and ask for permission next time.