I'd want answers to these questions:
1. Is this a type of concert where seating is assigned? My son is an audio engineer and he has been the sound tech at concerts where tickets are sold and seats are assigned, like at a professional baseball game. He's also done concerts where there are no assigned seats, and the crowd fills in the venue on a first-come-first-served type of thing, and those often can get very out-of-control, since the crowd often surges to get closer to the stage. If your daughter's concert-in-question is at a major arena with assigned seats, that might be a point in favor of her going. No assigned seats? Standing room? A point against her going.
2. What type of concert? Are the performers the type who frequently incite the crowd and jump into the crowd, or is this a more toned down fun concert?
3. What is the older brother like? What are his rules for driving? Might he stop and pick up a few more pals on the way? Is it legal in your area for drivers under 21 to have riders in the car? How long has the brother been driving?
4. Who's paying for this concert? Is she chipping in or paying it all out of her own money? Or begging you to buy her ticket?
If this concert is a fun group of performers or singers, I might consider allowing a 14 year old girl to go, but only: with the rules made crystal clear (no leaving the venue, having a cell phone and not using the cell phone to take pictures but stashing it securely in case of emergency), and I'd drop her off and pick her up at a carefully pre-arranged spot. I'd tell her that this will set the stage for future concerts and events. If she loses her phone, doesn't come out to the arranged spot afterwards ON TIME, smells like booze, then it will be a very very long time until the next concert. And of course this is all dependent on the type of girl she is. Is she reliable? polite? responsible? trust-worthy? getting good grades? helpful at home? If you're always wondering where she is, and if that little trip to the mall last month turned out to be a cover-up for the fact that she and her BFF actually were at a boy's house without your knowledge or permission, well, no concert then.