My Parents Reneged on a Conversation about college. Now I am SOL...?

Im going to a college locally because my parents told me that if i stayed local and stayed home.. They would convert the upstairs of the garage into an apartment.. It was already a family room with a kitchen and everything... They also told me that i would get a new car. Ok i understand that my parents have to work out some finances to get me the car and ill wait for that. But the reason I cant move into the family room is because my sister doesn't want to give up her bathroom that connects to it. What should I do... Just move all of her stuff out of her bathroom and into my old one. Or just should i move into the family room and use another bathroom in the house that is in the main living area and piss off my parents. I wanted to live in a dorm but it is too late now to get one... WHAT SHOULD I DO!