It may be rare that a stepparent is awarded custodial rights in any state, including here in Washington. However, it does happen under extenuating circumstances. The State of Washington recognizes what is termed a "de facto" relationship and has, since an important Supreme Court ruling in 2004, awarded custody to the party or parties which would serve the best interests of the child, regardless of biological or psychological relationship
In terms of stepparents' rights, a de facto parent is essentially someone who can be shown, through convincing evidence, to be the primary caregiver for a child in both a physical and financial sense. A court will look at factors including the status of the biological parents and whether the potential "de facto" parent shares a home with the child. In addition, the court will consider whether the "de facto" parent acts as a parent without any expectation of compensation, and how long that parent has been acting as a parent in terms of forming a lasting bond with the child.
Some of the extenuating circumstances for which may cause the court to award custody to a stepparent may be if both biological parents have been deemed unfit, or if both were deceased. Also, if a stepparent legally adopted the child during the married years, then they may be given custody consideration over a biological parent.
It can prove to be extremely detrimental to a child to be removed from the care of a stepparent and placed with a biological parent whom they do not even know. However, some courts may still choose to push a biological relationship if a strong enough case for the stepparent is not presented. For this reason, a stepparent may find it helpful to work with an experienced family law attorney to build a strong case for gaining custodial rights or visitation rights.