My gerbil is in lots of pain !!!?

Ok, so yesterday my evil little brother, decided to go in my room yet again! And he obviously tried to pick my gerbils up by the tail. When I came upstairs one of them was on my futon and the other sitting still in the cage. Anyway the one sitting still is my older gerbil Midnight, when I picked her up I saw the poor baby's whole tail had been ripped off, nothing but bleeding, raw cartilage left, I'm not to worried about that because I know that the cartilage will fall off in a few weeks, but thus morning I noticed that one of her hind legs are curled up, limp and cold, I think it's broken, she's limping around on her other 3 legs. My parents are upset, and they say she's dieing, my dad wants me to take both of them back to PetSmart today, and my mom wants me to bury them in the backyard today !yes alive! I know they are soooo horrible, they are always calling my gerbils rats and those things. Anyway, I need some REAL advice soon, should I put Midnight out of her "misery" or if not what do I do about her tail and leg? My other gerbil Sunshine is fine except for a small missing tuft of fur from her tail, she is licking/cleaning Midnight's tail and sleeping beside her. Please help !!!!!