AITA for eating my sister’s wedding cake before her wedding?

My sister is getting married next weekend, and she ordered a very expensive custom wedding cake.
The cake was delivered to our house yesterday because her venue doesn’t have refrigeration until the day of the wedding. It’s a five-tier cake, and she specifically told everyone not to touch it because the bakery had already warned her that moving or opening it too much could damage the decorations.
Well… I came home last night and saw it in the fridge.
I was starving.
And before anyone says anything, I genuinely thought there was a piece missing from the back because the box wasn’t completely closed.
So I opened it.
It looked completely fine.
I took a tiny piece from the very back of the bottom tier because I figured nobody would notice.
Except it was REALLY good.
So I took another piece.
Then another.
At some point, I had basically eaten a slice.
I put the cake back and thought that was the end of it.
This morning, my sister opened the box and immediately noticed.
She started freaking out.
Apparently the cake has a very specific design where the bottom tier is supposed to have a smooth white finish, and there’s now an obvious chunk missing from the back.
She asked everyone who did it.
I admitted it.
She started crying and said I had ruined one of the most important parts of her wedding.
I told her I was sorry and offered to pay for a replacement tier.
She said that wasn’t the point because the cake was custom-made and the bakery is fully booked, so they probably can’t recreate it before the wedding.
My mom thinks I’m an idiot but also says my sister is overreacting because “it’s just cake.”
My sister told me I’m selfish and that I always treat things that are important to her like they’re jokes.
I honestly didn’t think it would turn into this huge thing.
I also feel like if the cake was that important, she shouldn’t have left it in a fridge that everyone in the house uses.
But maybe I’m just making excuses because I was hungry.