The difference between Brethren and Brother

Brethren is also adjective with the meaning: of or akin to.

Brother is also noun with the meaning: son of the same parents as another person.

Brother is also verb with the meaning: to treat as a brother.

check bellow for the other definitions of Brethren and Brother

Brethren as a noun (archaic):
Brethren as a noun (figuratively):

the body of members, especially of a fraternal, religious or military order

Brother as a noun:

Son of the same parents as another person.
Brother as a noun:

A male having at least one parent in common with another (see half-brother, stepbrother).
Brother as a noun:

A male fellow member of a religious community, church, trades union etc.

Examples:

"Thank you, brother."

"I would like to thank the brother who just spoke."
Brother as a noun (African American Vernacular English):

A black male.
Brother as a noun:

Somebody, usually male, connected by a common cause or situation.
Brother as a noun:

Someone who is a peer, whether male or female.