Whence the Title?
Is it because Pope Francis is very conservative, and so he avoids inclusive language? Noooooo. Is it because Pope Francis is trolling the liberal media, just trying to get a rise out of them? Ye–Noooooo. Is it because the title cuts off the rest of the phrase: “All Brothers and Sisters”? Nope. And I notice that Human Fraternity, which is said to be related to the encyclical “All Brothers” uses the phrase “brother or sister” and “brothers and sisters” six times, never “all brothers” alone. So here is the most likely explanation: it’s a quote from the Bible
[Matthew]
{23:8} But you must not be called Master. For One is your Master, and you are all brothers.
{23:9} And do not choose to call anyone on earth your father. For One is your Father, who is in heaven.
{23:10} Neither should you be called teachers. For One is your Teacher, the Christ.
{23:11} Whoever is greater among you shall be your minister.
{23:12} But whoever has exalted himself, shall be humbled. And whoever has humbled himself, shall be exalted.
That’s why the encyclical is titled “All Brothers”, probably.
Ronald L. Conte Jr.
the reproach of Christ 
Dear Ron,
The title, “All brothers” (tutti fratelli) is from an admonition of St. Francis of Assisi according to Vatican News:https://www.vaticannews.va/en/pope/news/2020-09/all-brothers-pope-francis-to-sign-new-encyclical-on-oct-3-in-a.html
Pope Francis, as we know, has a deep devotion to St. Francis of Assisi. This will be the Holy Father’s second encyclical whose title comes from the founder of the Order of Friars Minor.
Robert, Thanks for that correction. From the link you provided: “The title, whose official English-language version has not yet been released, is a reference to the writings of St. Francis: “Let us all, brothers, consider the Good Shepherd who to save His sheep bore the suffering of the Cross” (Admonitions, 6, 1: FF155).”