What is the female equivalent of brethren?
There is a word: sistren.
Both brethren and sistren were used in Middle English (12th to 15th centuries) simply as the plural forms of brother and sister. From about 1600, brothers began to take over from brethren (Shakespeare used both), except when referring to fellow members of a religious community, or a society or profession. It still has this meaning today, and you may also find it used in ironic or humorous contexts (e.g. our brethren in the popular national press).
Sistren, on the other hand, had fallen completely out of use by the middle of the 16th century. It has recently been revived, typically by feminist writers, with the new meaning 'fellow women' (e.g. Lead singer Beth starts out most shows with several shout-outs to her sistren). This use is not yet well established in standard English.