Can a step parent be sued for a step child's medical bills?

In many states, yes!

20 states have direct laws regarding the duty of step parents to support their children.

Additionally, most other states hold a Doctrine of Necessaries that makes step parents liable for basic living expenses such as food, shelter and medical bills of their spouses, children and step children.

In states that use this doctrine, while one spouse is not automatically liable for general debts incurred by the other spouse, such as credit card debt for non-essentials, a spouse can be held liable for the spouse and children's medical expenses and even the portion of credit card debt that can be traced to things like groceries, clothes and doctors' fees.

This situation is especially relevant when a custodial natural parent who has remarried loses his or her job and is unable to pay for basic bills. Even when a second, non-custodial parent may also be held liable for a portion of the children's bills, the step parent (living in the same household) can be held liable for the custodial parent's portion of those bills. In community property states, the liability is even more clear cut, allowing creditors to garnish a step parent's wages directly for severe outstanding debt when unable to collect from his or her spouse.

Additionally, a step parent who is providing health insurance for child (ie carrying children on an employer's policy) can also be held responsible through contractual liability, as he or she is the legal guarantor for any charges incurred while using this insurance.