Ungrateful Niece - should I let her suffer the consequences?

Let me try to briefly explain the situation. My niece lives with my grandmother. She's about to graduate high school. No car. My grandmother is retired, no money, can't afford to get her one. Her father lives there as well, in and out of jail for drugs, sorry. Her mother is the same way. Well, I convinced my mom to go in with me and if we could find a car for $1000 to split it to help her (and my grandmother out because she never hears the end of it) get one. I found a GREAT running car. Told the people selling it her story, offered it to me for $500!!! It's a 1993 Crown Victoria. I drove it and everything. Nothing seems wrong with it. Well - Brianna's dad has $1000 right now from refunds. and is about to get $800 in a couple of weeks. This is the part that pisses me off - Brianna doesn't WANT the car I found. It's not *good* enough for her. ???!!! You know!? So - Brian (her dad) want so to wait til he gets his other check and find something for her himself. Only thing is ..to be cont.