What a fabulous photo!
Hats were at their widest around 1911, and the v-necked blouse came into fashion in 1913/14. So, as you can wear a style that is a little out of date, but you cannot wear one that hasn't been invented yet, this has to be after 1913/14.
But as the hat is still pretty wide, and what a hat it is

...I'd put this right at the start of the v-neck blouse. Call it 1914. So the younger woman.
Cheers,
China