If nuns are called “sisters” why not call priests “brothers”?
The male equivalent of a Nun is a Monk.
The male analogue of a nun is not a priest, but a monk, and in the Latin West, monks are called “brother” just as nuns are called “sister”.
In the Orthodox East, both priests and monks are addressed as “Father”, and nuns are addressed as “Mother”. Deacons are addressed as “Father Deacon [Christian name]”, but written of simply as “Deacon [Christian name]”.
Because they take different vows.
"Brother" is a title given to the members of a religious order. It typically implies that the one using it is a person who has taken vows of obedience, celibacy, and poverty, but who has not been ordained to Holy Orders.
"Father" is a title exclusively reserved for those who are ordained to the priesthood.