Am i being a jerk here or am i right?

I went over to one of my brothers yesterday for dinner. My niece is 2 years old and literally snacks all day long. Barely any real food just puffs and crackers and sometimes a banana and she expects you to share everything with her or give it to her it was dinner time and she was wanting my pie. She didn't eat any of her dinner but wanted my pie. I said no. I told her that she doesn't get whatever she wants and that's a life lesson for her. My sister in law yelled at me for being selfish and i told her she needs to learn to eat her own food and to not mooch off others. Nobody is expected to share all the time. She grabbed my brothers icecream cone from his hand and ate it.

So im the jerk for trying to teach manners and trying to promote real food eating and not begging.