Back child support payments statute of limitations. Finding out after 18 years that your daughter is not yours.

This is difficult. I think the easy answer is "yes," he is still obligated to pay child support for all 3 until and unless the Order of Child Support can be modified or terminated.

Regarding the 18 year old, can he prove who the real father is? If so, I still think he can't bring any civil action. The court is normally not going to relieve an obligor/payor of child support unless there is a substitute obligor/payor to take over, and it is doubtful that you can go backwards. If the children were under 18, I would suggest filing a "Petition to Dis-establish Paternity/Parentage, Determine Parentage and Terminate Child Support", but even then, the court may find the long-term bond does not justify this requested relief and dismiss the lawsuit. So, you can imagine that with a child over 18, the issues becomes more difficult because I predict the court would not even entertain such a Petition because it would be moot.

I have never seen a lawsuit where someone has sued their ex or the biological father for overpayment of child support. You could call DSHS in Olympia and ask who the prosecuting attorney for child support in your County is to see if they would entertain filing a parentage (i.e., paternity) action against biological father and mother (at this late stage), during which perhaps the State could go back 5 years under the Statute to establish biological father's obligation and therefore, seek to relieve your brother of those 5 years back. However, again, I doubt such a case would survive a motion to dismiss for lack of jurisdiction or other reasons due to the child's age.

Side note/query: Does the father want it established that he is not the father of the 18 year old? Does he want her birth certificate to reveal that he is not the father?