My sister started crying, should I feel bad?

My sister LOVES apple pie. I went out and I bought everything for apple pie yesterday. We were going to have an all nighter party. I came home and made the apple pie. Everything was fine. But she always finishes everything really quickly so I politely asked her to leave some and not take anymore after two reasonable slices (max). This is because my mother and father had gone to bed when I made it and I wanted to make sure that they would have to as well. She said that she wouldn't. We stayed up a little later, than we went to bed. Or so I thought. I was asleep and at about 2:00 am, I heard a noise in the kitchen, so I went out there. there was my sister taking a huge slice of apple pie. i asked her to leave it alone and there she is stuffing her face saying that she's not going to finish it. i asked her again to leave it alone and she said she would so i went to bed. at about 4:00 am, the same thing happened. I was pretty upset then because she kept on lying to me so I waited until she went back into her room and then i wrapped it up and hid it. Mind you, I didn't take any, because I felt it would just contradict the whole thing. About half an hour ago, she came into my room and asked me where the brownies were. There was a whole seen of me telling her that I ate them, they were in the supermarket, and whatever and her keeping on asking. Then she takes my laptop, which I'm trying to do my homework on and starts using it. Then I get pissed and snatch it from her. Then she breaks down and starts crying about how "i;m constantly mean to her" and how "I don't respect her" and that "she tries to be a good sister". So all I do is ask her how I am constantly mean to her, an example. She sits there and tries to think of one way and she cant. I'm normally not like this but I'm tired of everyone not being able to get ANYTHING because her philosophy is that if someone leaves it there, you can take the whole thing. Maybe its just that other people are being considerate and making sure that the other person got some. I told her that I respect her, because I do and then she's like "STOP LYING TO ME!". I mean, I was lying? Who was lying about just leaving it there. I actually don't care about the apple pie, It's just that she is always inconsiderate; she never thinks about the next person. My dad was upset because she always demands for "please and thank you", but she never actually says it herself. I just wish that she would think about the next person adn think to herself" okay, I can take some but just leave a little bit so EVERYONE can enjoy it." I'm beginning to think that I should just tell her what I just wrote because clearly she doesn't notice it. I know it was wrong of me to not tell her where the brownies are, but I just went away for 4 days and there was a whole loaf of hard dough bread and it was just her and my dad. my dad doesn't like hard dough bread, so i knew when I came back and there was none left, that she finished it. But I don't feel bad about the whole scene, should I? And how could I mention it to her that my dad, mom and I would appreciate if she was just a bit more considerate?