AITA for calling my sister a jerk for refusing to share her snack even though she didn't eat it?

&#x200B;<br><br>I (16m) went to the mall with my sister (14f), brother (18m) and his girlfriend last night. While we were there, they decided to split up for an hour and explore the mall on their own. My brother and I went to GameStop and my sister and her girlfriend went to the food court. <br><br>My sister and her girlfriend were finished shopping before my brother and I were so they went to the entrance and waited for us. My sister had a starbucks coffee and a cookie from the food court. My brother and I were finished shortly after so we met up with my sister and her girlfriend at the front entrance.<br><br>My brother and I were both famished and when we saw my sister with a cookie we both asked if we could have a bite. She said no and put the cookie in her purse. I asked why she put it in her purse when she was going to eat it and she said she wasn't going to eat it. My brother and I asked why she wouldn't share and she said she was saving it for later and we couldn't have any.<br><br>She had the cookie for the rest of our trip and when we got home she still hadn't eaten it. I don't know why but to me it made sense to take a bite when I was hungry and save some for later. I asked her again why she wouldn't share and she said she was just trying to save it and it didn't matter because we wouldn't have liked the flavor anyway.<br><br>She said I was being a jerk for trying to take her cookie and not minding her decision but I think she was unreasonable for not letting us take a bite.<br><br>AITA?<br><br>Edit: I think there's been a misunderstanding regarding the cookie. The cookie was about the size of my fist, not loaf-of-bread sized. When I said bite, I meant one bite and no I don't normally take a bite of people's food without asking. I also didn't say I liked coffee. I said there was nothing wrong with coffee and if it was good to her then it tastes fine. I know I'm not supposed to like coffee because I'm 16 but I don't like it anyway. I was just trying to say that coffee tastes fine.  I don't know how to explain it better than that.  When I said she was a jerk I didn't mean to call her a jerk I just meant she was being unreasonable. Yes, it was her cookie and she could do what she wanted but she wasn't even going to eat it. <br><br>Edit 2: I just want to add that I didn't say she was a jerk but rather she was being a jerk. There's a difference.  A jerk is someone who is consistently annoying where as being a jerk is acting like a jerk in a situation.  I apologize if it came off like I was calling her a jerk.