IANAL, but frankly I find a lot of the comments & suggestions here baffling. A few random notes from my own perspective:

Most laws pertaining to the consumption of alcohol refer to a "minor in possession" of alcohol. If your brother orders it and drinks it, you're never in possession of it.

I wasn't able to locate a Tennessee statute, but here's an example from Texas. According to Sec. 106.05 of the Texas Alcoholic Beverage Code:

(b) A minor may possess an alcoholic beverage:

(1) while in the course and scope of the minor's employment if the minor is an employee of a licensee or permittee and the employment is not prohibited by this code; 

(2) if the minor is in the visible presence of his adult parent, guardian, or spouse, or other adult to whom the minor has been committed by a court;

In other words, in Texas the laws specifically states that a minor can possess alcohol in the course of their job (say as a waiter), or they can possess it when in the presence of their parents guardian or other authority figure. Your brother, being older than you, would more than likely fall into the "guardian" category in this case. But again, you're never physically possessing the alcohol so it should never even get to this point.

Although this specific example is for Texas, I'm sure you could find similar wording wherever Tennessee spells out its laws regarding alcohol.

Anothing thing to keep in mind is that these laws also make it a crime for a business to sell alcohol to a minor. If you got in trouble for paying for your brothers drinks then the restaurant could get into equal trouble for providing your brother with the drink. The restaurant won't bat an eye at this, so don't worry about it.