I got out my magnifying glass and studied her brooch after scanning the photo.
Look at the two adults at the right of the photo below--Tillie Jacobs Mahler and her husband, Meyer Elias Mahler. (Thanks again to Cuz Lois for identifying this photo.)
The mystery lady's pin clearly shows Tillie and Meyer, taken in the same studio at the same time as the big group photo.
The family portrait was taken around 1900, judging by the ages of the children. Knowing that Meyer died in 1910 helped Lois and me guesstimate the timing.
Other, later photos in Cousin E's batch show the mystery lady wearing the same brooch.
In the photo below, taken at the site of Meyer Mahler's grave, the now much older mystery lady has the brooch pinned on.
No magnifying glass needed this time since I blew up part of the photo and put it below.
Wait! There's more . . .
At bottom, a cropped version of a photo my cousin Ira lent me several years ago--a photo we know to be of my great-grandma Tillie Jacobs Mahler.
But we never noticed the brooch, which was not sharply defined in this photo from the 1940s.
Now we know Tillie kept this brooch as a memento of a happy family time.
She wore it for four decades after her husband's death, until she died in June, 1952 and was buried next to her beloved husband.