After Dark: A Vamps Novel by Nancy A. Collins

What's on the back of the book:
It's the most important event of the season in New York's upper-crust vampire society. The Rauhnacht Ball has everything you need for a night of scandal and intrique: gala dresses, stolen boyfriends, forbidden love . . .
Then a team of vampire hunters shows up and ruins everything! The surprise attack by Van Helsings is the worst gate-crashing ever.
In the aftermath, half sisters Lilith and Cally scramble to regain their footing at the exclusive Bathory Academy - and in the world of high fashion, where both have star potential. When new suitors turn the romance up a noych, It's almost enough to make the half sisters forget their intense rivalry. Almost. But shared blood between vampires is not easy, and before long Lilith and Cally find themselves face-to-face again.

A piece of the book:
Finally out of the tear gas, Cally staggered across the stone terrace toward the wide, curving stairs that led to the gardens below.
"Fire!"
Cally looked up just in time to see dozens of crossbow arrows flying toward the terrace. She ducked, putting a marble replica of the Venus de Milo between her and the deadly rain. As she watched from her hiding place, she saw one of the other guests jump atop the balustrade's railing, instantly shapeshifting into his winged form.
With a beat of his eight-foot wings, the transformed vampire shot up into the night sky in a desperate attempt to escape the Van Helsings' crossbows before they could reload. At first it looked like he had succeeded, but then a shadow soared from the roofline of the building.
With just a few beats of it's own leathery wings, the gargoyle easily overtook the feeling vampire, who screamed as the beasts slashing talons destroyed his right wing. Unable to maintain balance, the vampire spun out of control and crashed a hundred feet down into the hedges that ringed the gardens.
Cally's guts tightened as she listened to the gargoyle shriek in triumph. Things had just gotten a whole lot worse: both the Van Helsings and their pet were out for blood.