The obvious first steps are unavailable to us: that is, asking the aunt doesn't work because she is dead and asking her other siblings doesn't work because they are either dead or alive and not talking to us and even when they were talking to us they wouldn't talk about this particular person. Another obvious step would be asking the offspring of this uncle, but it took a long time to find that person and our relationship is super tenuous at this point.
The next step is just to see if the name is on the birth certificate, hence, the mail from Washington State. I was a bit surprised to find out I could even get the record (I'd been missing on so many other states for people who were long dead). I warned my sister not to expect much; I was expecting exactly what we got: no name in the father field.
But I'm still amazed that there are no obvious limitations on vital records in that state and seriously contemplating sending in a ton of requests to get records before someone decides to make them every bit as annoyingly unavailable as a lot of other places. OTOH, their Digital Archives already have a ton of marriage licenses, so maybe they're just going to be reasonable and I can wait and cherry pick the ones I really want.
Hard to say.