You or your sister could be sued by the person whose car was damaged. Sounds like a small claims case where it's like Judge Judy and no one has a lawyer. If the person contacts you or your sister for damages, explain that you have no insurance and how much $ the person wants and ask to see the estimate. If it's something you can pay, then contact a lawyer to prepare a Release of All Claims. Most lawyers including me would do it for free or a small fee. If you can't pay and you lose in court then the person can try to collect from you in various ways like garnish your wages etc. You may then want to speak with a lawyer depending on how much you owe. I would just try to settle with the person if you are contacted.
You can indeed be sued. If the person whose car was damaged sues, it will probably be in a small claims court and you could be ordered to pay some damages (or your sister can be ordered to pay). If you do not pay, the person winning at trial can pursue the dollars through your bank account or wages.
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.
Other party likely will claim from her own ins for prop damage, and possibly uninsured motorist injury claim. Her ins co can then sue you and your sister for reimbursement. if dmv notified, you can get your license suspended for having no insurance.
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