There was a good deal written in the mid-1800's about "consanguinity"
-- the marriage of close relatives -- and what was seen as the
detrimental effects on the offspring of such marriages.
Most of the information covers the topic either generally, or is
specific to a particular area, usually in the context of a legislative
proposal that is being debated.
Although no articles turned up that were specific to the public debate
in Washington state, I did find this mention of the Washington law in
the proceedings of the National Conference of Charities in a
publication, "The social welfare forum". published in 1897:
In particular, a section on "State Regulation of Marriage" can be seen here:
and on the following page:
which discusses the "doctinre of consanguinity" and notes the
differences in the laws of several states, including Washington (next
to last paragraph on the page).
This article (which I referred to in my earlier comment, above) is
from 1858, and discusses a Kentucky report on the topic of marriage
I trust this information fully answers your question.
However, please don't rate this answer until you have everything you
need. If you would like any additional information, just post a
Request for Clarification to let me know how I can assist you further,
and I'm at your service.
All the best,
search strategy -- Searched Google and several historical databases
for combinations of the terms: