Of course, you're quite right. I was reading too rapidly and goofed. Isaac and Lucy Gertrude Arthur Nidiffer had 11 (by my records) children. Sarah Elizabeth was number 5, and not quite two years older than Felix (my great-grandfather).
In my notes, I have a listing for an Eva Nidiffer, who married a Bird. Fay Arellano's notes do not list her with Isaac's family, yet for some reason I have her with Felix, Sarah et al. I think I recall hearing of her from Grandmother (Annie Lula Nidiffer Allison).