Why Did I Really Take That Cookie?

So here's the thing, last week, my older sister was baking these amazing cookies for a school bake sale. She made like fifty of them and left them cooling on top of her dresser while she went out to practice with the softball team. Now, as you might guess from me being the youngest in our family, I'm not exactly known for my strong willpower when it comes to sweet treats, especially home-baked cookies. So here's where it gets confusing: I had been planning on waiting until after school to try one or two of those cookies. But something weird happened, I just kept going back and forth in my head about whether taking even a tiny taste would really be so bad. One minute, I was thinking that if she made fifty cookies, surely she wouldn't miss just one. Then the next moment, I felt guilty and promised myself it could wait until later. Yet somehow, by lunchtime, I ended up with crumbs all over my hands from stealing one of those precious cookies. I know deep down it's not the best thing to do because now my sister is going to need to make another cookie or two for the bake sale since she'll probably notice that one went missing. But here’s what bothers me, I don’t think I did this just because I love her baking so much; there seems to be something more at play here, like a weird psychological thing where the 'forbidden' made it seem extra tasty and irresistible. So, am I just being too hard on myself for one little cookie? Is there any way that taking food meant for a fundraiser could actually feel justified in certain situations?