I'm not going to save my brother a seat
Can you tell me why you don't want to save a seat for your brother?
He is a lazy and forgetful person and it makes me sick
Having consideration for other family members is important and this includes saving a seat for them if you have the opportunity to do so. It would be hurtful for your brother not to be able to sit in the same area as you and the rest of the your family. If you are upset with your brother's past behavior you should discuss it with him rather than try to get petty revenge on him by not saving him a seat.
WTF. It's my family and I will treat them how I want. I don't care about him. He thinks he's so responsible, but he isn't.
Your brother would want to be near you for the event, and not reserving a chair for him will make him feel excluded. Set aside your judgment of him and do the right thing here.