"1920 US Census - District 383, Dallas, Polk County, Oregon, USA"
I found the twins in the 1920 Census. =) I'm still on the trail of Aunt Harriet's twin, Elvie, so I can possibly get a photo of her grave. I've searched and searched on Ancestry.com and surely should have found this by now! It turns out that you can view an entire county's census records without finding them in a search. I didn't know that and just stumbled on it the other day. So I took some time out to look through the records of the towns that were noted in the family tree.
Looking at the information on the census form, it's no wonder I couldn't find them! Harriet is listed by one of her middle names, Loretta. Elvie is listed as... Anetta? (can't read the cursive writing very well). Their dad Frederick is listed as Fred, Uella is listed as Ella, and I think they got Ray and Roy mixed up or something and crossed out and rewrote it so you can't tell which one it is. Heck, maybe they were both there and the census lady got confused. It's hard to tell.