I'm not going to give my dad's girlfriend's little brother a meatball.
Why would you not give him a meatball?
Because I think he's had a enough.
If someone is hungry then you shouldn't ignore them. Family is expected to look out for each other especially if it's something that you can help solve easily.
It's not my job to give him anything.
It isn't your job, but it would still be nice of you to let him have a meatball, because he's important to your dad's girlfriend. Plus, he is probably still hungry, and you shouldn't let someone go hungry if you can help it.