This portrait of Mary McClain was given by her to Mom/Florence McClain at Christmas, 1929.

Mary was almost exactly four months younger than Mom (June 6 to October 7). But I suspect Mary’s “late” birthday put her a year behind Mom in school—meaning Mary graduated in 1929 while Mom had graduated in 1928. This portrait, then, is most likely Mary’s high school graduation photo.
This did not put Mary behind Mom in other ways, though. She married Carl Rundback April 29, 1933, just over two months before Mom and Dad married. (Interesting bit of trivia: Carl was born just one day before Mary.)
I believe Mary resided in the vicinity of Paxton, Nebraska her whole life. Carl and Mary’s spread there was called the Rundback Ranch. Evidently they took that over from Carl’s parents.
Mary barely beat Mom in longevity. She passed away in August 2003 (versus Mom’s September 2002). Carl passed away in 1981, so Mary ran the ranch for a long time by herself (possibly with the help of some or all of her three sons, Morton, Kent and Larry).
The ranch was liquidated in 2010.
Mary and Mom were not only cousins, but lifelong friends, from what I can tell. It is remarkable how close together the major milestones of their lives were.