I want to throw my baby bro a party for his birthday at a hotel?!

You definitely be the one charged with aiding to the delinquency of minors. Have the party in your back yard and no booze, If you do have it in a hotel and there is damage you are liable. Another point look at the card on the back of the hotel room door. On it will be the actual price for that room,
You might get the room for 100 0r 150 dollars. but the actual price of that room might be 350 a night.
If that room is damaged the hotel has the right to charge you that 350 price posted on that card for everyday that room can not rented while being repaired. And remember the noise coming out of that room will be called into the front desk from some other guest room on that floor.

Worked in a hotel and have seen people do what your planning and it cost them a fortune.

This is a really dumb idea.
Yes you will get in trouble for knowingly supplying alcohol to minors.
It's a felony and someone will call either from the hotel or another guest.
Hotels frown on their rooms being used for parties, especially ones that involve underage drinking. If you break anything or stain anything they can rack up as much as they want on the credit card you supplied when you signed up for the room.
Even if you are not there, the room is in your name and you will be busted.

if there is damage to the hotel then yes that will be your problem, and idk about the states.. but i know in canada as long as theres someone of age there watching then its fine, i've had many parties at my house and when the cops come and they see that there is someone of age they dont care

Here's a smart idea, I'll use my credit card to secure a room in which a group of high schoolers, without any adult supervision (not even myself), will get drunk and rowdy. And my credit card is securing all this. Again, smart idea.