What is the opposite gender of brethren?

Answer 1

According to the Oxford English Dictionary 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. Around the start of the 17th Century, 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. Even this use is now rather archaic (though groups such as the Plymouth Brethren keep it in use). Sistren, meanwhile, fell completely out of use by the middle of the 16th century, but has since been revived (and used almost exclusively) by feminist writers.