Okay,

get arrested for what? not making a car payment? your story isn't very clear

that is just a scare tactic to get people to pay immediately. if your mother signed a contract for the purchase of a car, there are RULES spelled out in that contract. usually though, the car is impounded -- without arrest.

i think i'd be talking to an attorney about having a cop show up at the door.

and does your almost-18 sister contribute to the bills?

well first off.... even IF your mom gets arrested, the courts will not allow two minors to remain in a house alone. there will be arrangements made to make sure you and your sister have a guardian.

secondly, your sister has a responsibility to the family to get off her ass and contribute to the bills. i think all children capable of working SHOULD work towards the communal funds.

third, i definitely would talk to your mom about your concerns. if not your mom, find a responsible adult that you can talk to about these issues. there are plenty of people out there to help you -- a counselor, a librarian, a neighbor, a family member, etc.

i dunno where you would go. i suppose the closest adult relative within reason.