fraternal
Related to brothers or brother-like friendships and bonds.
Fraternal means relating to brothers or having the kind of bond that brothers share. The word comes from the Latin word for brother, frater.
When people describe a relationship as fraternal, they mean it has qualities we associate with brotherhood: loyalty, camaraderie, and a sense of being in something together. Members of a fraternal organization aren't actual brothers, but they treat each other with the same loyalty and support that brothers ideally show one another.
You might also hear about fraternal twins. Unlike identical twins, who come from the same egg and often look very similar, fraternal twins come from two different eggs and can look as different as any other siblings. They just happen to have been born at the same time. Fraternal twins can even be different genders, while identical twins are usually the same gender.
When coaches talk about building fraternal bonds on a team, they mean creating the kind of trust and connection where teammates look out for each other like family.