The phrase "blood brother" refers to a very close friendship or bond between two or more people. It is often used to describe a bond that is as strong as if the people were related by blood. Full definition
For we humans are all genetically linked; we share nearly all of the DNA formula peculiar to our species, and we are already bonded as blood brothers and sisters. (religion-online.org)
In some ways they remain more connected to me than my own blood brother who I have less contact with. (bookbrowse.com)
And at the very end, my mother had my youngest blood brother. (educationnext.org)