Legal Question in Family Law in Washington

Generally speaking, no. If the girlfriend of your ex has children from her she receives child support, it is double dipping for your ex to claim that they're also responsible for those children. However, in situations where they legally become your ex's stepchildren, there is something known as a step parental duty of support, which only gets triggered in limited circumstances, such as a dissolution of marriage where people with stepchildren may have to pay temporary support for the stepchildren. However, since your situation does not pose that scenario, it does not apply, and in any case, for purposes of determining what child support should be for your children, I do not believe your ex can claim the other children to receive a downward deviation.