i think you are both being a little unreasonable
just because you are out of a job does not mean you should live for free. If you were in your own bedsit, your landlord wouldn't accept no job as a reason not to pay rent. Your mum is under no obligation to house you at all, and can in fact give whatever benefits she likes to your brother and dosent have to give them to you or your sister. On the other hand, you cant pay what you dont have can you? I think it would be a really good idea to sit down with her and work out what she wants from you .... maybe she has money problems and has built her courage up over weeks to ask you for some money? Do you do anything all day? It can be hard for someone who's working to come home and find a person who isnt working hasn't tidied up, cleaned the house, cooked the dinner etc. Even if she isnt working it would be reasonable to expect you to cook and clean even if you were paying rent. i would also say its not up to me to tell you what to do, but cant you get rid of the burden of the car (as you will actually be getting less than it costs, not including tax, insurance and petrol) and get a mundane job in macdonalds, or a shop, or cleaning till you find what you want?