A summons for debt collection was given to my brother at my parents house, but I no longer live there. Is this a proper form of service in Washington State? It says I have 20 days to respond, but is that even viable if this wasn't a proper service?

Did you ever live there? Did you ever give your parents as references?

Dodging a law suit by trying to evade service will never work. The fact is you know about the suit so deal with it. If you ignore the summons on the pretext that you never got it, you will find yourself with a default judgment.

If you try to claim improper service and the court asks if you are in contact with your parents and brother then service will be deemed valid.