Declaring Moslems to be "brothers and sisters" renders the whole Catholic Church moot. What part of "I am the Lord your God, and before me thou shalt have no other gods" does Francis not get.
They are our brothers and sisters but not because they are Muslims.
One may mean "brothers and sisters in that we are all one part of the human family, sons of Adam according to the flesh."
(This makes us brothers and sisters because of our relationship to Adam, our common ancestor.)
The Bible refers to God as the "Father [meaning Creator] of all" (Ephesians 4:6)
(This makes us brothers and sisters because of our relationship as creatures of God, our Creator.)
What we are NOT, is brothers and sisters to Muslims as born-again in Baptism or in the Christian Faith, since they do not share our Baptism or the experience of being born-again or the Adoption. They are not of the household of the Faith. They are not our brothers and sisters "in Christ". Nor would they claim to be.
So they are our brothes and sisters in some ways, but not in others.