Most (probably all) of the pre-made chicken coops house way fewer hens than what the manufacturer says, and only bantam size. They're not predator proof either. Have you thought about making part of your garage a coop?
I'm sure you know there should have been preparation before getting these chicks. Please don't get a poorly made coop that is too small for them because of someone else's irresponsibility. Your sister and mother should step up and take responsibility and help pay for a proper coop since it was...