Legal Question in Family Law in Washington

The only obligation the father has, is the same one he has had-to pay child support. You, as an uncle, have no legal duty to the children. The girls may have a say, but not the final word on where they will live, regardless of age. It is just that as they get more mature, their viewpoint has greater weight when a court decides where they ought to live, if the parents cannot agree sooner.

If mom loses her home, then yes, dad can file a petition to modify the parenting plan and child support order to reverse the situation and have the girls placed with him, at least temporarily, considering, as you have said, mom is otherwise homeless and unable to provide for her daughters.

If the girls have nowhere to go, and they are homeless, CPS will bring them to their father. If she says I don't want them, the state is likely to put them with relatives who can provide for them and child support would go to the relatives, otherwise, the children will be placed in foster care, and there is no guarantee they will be kept together and the dad's support will go to the foster parents.

Hopefully, between dad and the rest of the family, you all help your sister in law and her three girls out until she can get on her own feet again.