The difference between Brethren and Sisterhood

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

Sisterhood is also noun with the meaning: the state, or kinship of being sisters.

check bellow for the other definitions of Brethren and Sisterhood

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

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

Sisterhood as a noun:

the state, or kinship of being sisters
Sisterhood as a noun:

the quality of being sisterly; sisterly companionship; especially, the sense that women have of being in solidarity with one another.
Sisterhood as a noun:

a religious society of women
Sisterhood as a noun (feminism):

The idea of universal experience amongst women, regardless of other traits or factors. (Considered obsolete in third-wave feminism.)