|The stupid question is the question not asked|
Good suggestion Petruchio.
Personally though, I was completely distracted by your opening words (my fault not yours) which prompted me to investigate 'Sistren' a little.
I present here the results of my research (OK, I Googled for it), for the benefit of those brethren with philological interests...
There is a word sistren, though it has a somewhat different history from its male parallel. Both words 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 in 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, had fallen completely out of use by the middle of the 16th century, but has been revived (and used almost exclusively) by feminist writers.
From AskOxford.com (part of the Oxford English Dictionary), which - I've discovered - also has many other interesting articles. For example, the collective noun for a group of cats is a 'clowder'.
With best wishes,
In reply to Wildly off-topic re "Brethren and Sistren": Is 'Sistren' a proper word?