While seemingly improbable, they are by all accounts brothers. Refered to as Francis Sr and Francis Jr. Francis Sr being baptised in Tichfield, Hampshire, England 17 Jun 1619 and Francis Jr baptised 24 Mar 1635/36 at same location. I'm not an expert, but I believe the most popular reason given for this is that Francis Jr. was named in memory of his maternal uncle, whereas Frances Sr. had been named for the paternal side of the family.
Most of my data has been derived from the Littlefield Family Newsletter, published in Wells, Maine where many descendants of Edmund have resided for over 300 years. Info on newsletter and more can be found at www.littlefieldhistory.com