Buying insurance now will not cover the accident.
Hiring a lawyer won't help and will only waste more money. There are no magic answers.
"We don't know what to do". You will have to face the DUI charges, someone will have to pay the $1800. to get the car out and someone will have to continue paying the $10,000. still owed on the car. If Sister walks away from the loan, her credit will be ruined and if she is ever able to find an insurer in the future, it will be extremely expensive.
You and your Sister have some decisions to make.
Sorry, but You both made foolish decisions, now you will have to face the consequences