All that can presently (July 2023) be gleaned about The Kelly Sisters (Nellie and Flo) from sheet music covers and by Internet searching is that they were presumably American actress/singers who have been featured on just one work namely Rock-A-Bye My Baby Blues (1924) music by Billy Hill, words by Larry Yoell and published by Sherman & Clay Co., San Francisco, California, USA Their portrait on the left is taken from that same work.
Surfers are welcome to contribute any further information about these performers.