Frank B. Gilbreth and Ernestine Gilbreth Carey
This site, the Gilbreth Network, is actually focused on Frank and Lillian Gilbreth, the parents in Cheaper by the Dozen but does have some information on Frank B. and his sister Ernestine who wrote the books.
Some bare bones biographical information is available at the IMDB site for both Frank and Ernestine.
Ernestine Gilbreth Carey's Wikipedia entry.