My dad bought me and my brother some ice-cream and I'm positive more than half, if not all of it is going to be gone before my brother gets any.
Are you going to have friends over to have icecream as well or something or are you really just going to eat it all yourself.
My friend came by and ate a ton of it earlier so after I get some there likely won't be any left.
You still wanting to get your share of the ice-cream is understandable but it was your friend that ate most of it. That makes it your responsibility. You should try to be fair to your brother instead of being selfish and thinking about yourself.
WTF but I want my share to eat too
You probably should have thought about that before giving some to your friend. They are your friend and therefore your responsibility. It's not your brothers fault that your friend ate ice cream and he shouldn't be punished for it.