Honestly, if it weren't for snapchat, this wouldn't be news at all. I'm sure it's not the first or the last time kids have done something like this with a baby. The good news is that the baby is fine (and, realistically, wasn't ever in real danger, since you can't really argue they planned to leave her in there to either suffocate or destroy the contents of the fridge) and now mum knows not to leave the baby with these particular teens in the future.
MY teenage babysitter tied me to the dining room table leg with a skipping rope and only untied me because I was screaming bloody murder and she was afraid she'd get caught by her mom. If she'd had a cellphone, she SO would have snapped a pic of me to share with her friends.
It's just plain, ordinary barely-newsworthy stupidity. I think disciplining the girls should be left to their parents.