These Deadly Games by Diana Urban

Let’s play a game.
You have 24 hours to win. If you break my rules, she dies. If you call the police, she dies. If you tell your parents or anyone else, she dies.
Are you ready?

When Crystal Donavan gets a message on a mysterious app with a video of her little sister gagged and bound, she agrees to play the kidnapper’s game. At first, they make her complete bizarre tasks: steal a test and stuff it in a locker, bake brownies, make a prank call.
But then Crystal realizes each task is meant to hurt—and kill—her friends, one by one. But if she refuses to play, the kidnapper will kill her sister. Is someone trying to take her team out of the running for a gaming tournament? Or have they uncovered a secret from their past, and wants them to pay for what they did…
As Crystal makes the impossible choices between her friends and her sister, she must uncover the truth and find a way to outplay the kidnapper… before it’s too late.