Although it may seem obvious to you that you should not be held responsible, you will need an attorney to represent you regarding these charges. I suggest you contact an attorney quickly so your attorney can contact the DA's office and attempt to have these charges dismissed. You cannot be held liable for the actions of another person, unless you assisted that person in some way. If you wish, you may contact my office which is located in Orange County.
You question is a little confusing.
Are you being charged for a DUI? Or are they trying to get you to pay his debts?
You are not required to pay his court debts - if the question is whether you have to pay for damages done while driving your car.
Do you have insurance? Did he have insurance? You may have to sue your step dad if he won't take care of it.
Good luck with your situation. The above is just general information and not specific advice.
It isn't necessarily too late - talk to a lawyer right away or appeal the previous decision.
Good luck on your case, and if you need a DUI lawyer, check http://dui-help.com
Joshua M. Dale, Esq.