Whether or not a stepparent has a duty to support his stepchildren will depend on the location where you live. While traditional common law (judge made) rules indicated that a stepparent had no duty to support a stepchild unless an adoption had taken place, many state legislatures have since passed statutes requiring stepparent support under certain circumstances.
Stepparent Support Requirements
As a general matter, the duty to support a child belongs to his or her parents, even in the event of a divorce. As such, the income of a stepparent is usually not going to be factored in when calculating the child support obligation of the non-custodial parent. However, the courts also recognize that stepparents do have a relationship with their stepchildren by virtue of the marriage to the mother/father of the child.
As such, in many states, there are some requirements that stepparents provide child support. In some cases, those requirements do not kick in unless the custodial parents become unable to support the child. For example, under the Delaware Annotated Code Section 13, Chapter 5 section 501(b): "Where the parents are unable to provide a minor child's minimum needs, a stepparent or a person who cohabits in the relationship of husband and wife with the parent of a minor child shall be under a duty to provide those needs. Such duty shall exist only while the child makes residence with such stepparent or person and the marriage or cohabitation continues."
Other states that have statutes requiring a stepparent to provide child support include (but are not limited to) Iowa, Kentucky, Montana, Nebraska, Nevada, Utah, and Washington.
When a stepparent does have an obligation to support a stepchild, that duty only exists for the duration of the marriage. In other words, upon divorce, the stepparent will not have to pay child support or have any continuing obligations unless he has adopted the child or signed a marital agreement obliging him to do so.
Getting Help
Because laws vary from jurisdiction to jurisdiction, if you have concerns about your rights or obligations under the law, it is always in your best interests to consult with a lawyer for help with understanding the rules in your state.