The girl grew up in Texas. She's giving the UT (University of Texas, Austin) "Hook 'em, 'Horns" sign. The team is the the Longhorns. Texas Longhorns.
According to this link; http://www.foxsports.com/college-football/story/hook-em-horns-hand-gesture-harley-clark-100914 the gesture was instigated by a UT cheerleader, at a combination campus fund-raiser and football pep rally, about 60 years ago.