I ate all the left over food so now my brother doesn't have anything to eat
I hope you are joking. Why didn't anything stop you from eating all the left over food?
I didn't know that he would get so mad at me, but nevermind I'm already over it.
Taking all the food and keeping it for yourself is wrong when there is none left for your brother. Go to the store and stock up the fridge again for everyone in the house.
I'm on a diet...
It does not matter if you are on diet, you should still replace the food you have eaten so others can eat too.