In our church, we refer to one another as brother and sister. Sometimes, when referring to multiple people, we use the phrase brethren and sisters. Yesterday, someone said brethren and sistren then apologized for using a made up word.
It was a perfect opportunity for me to educate everyone and put his mind at ease. :)
Several centuries ago, brethren and sistren were the plural forms for brother and sister, respectively. After about 300 years of usage, brethren grew out favour and the simpler brothers took its place. Today, brethren is used primarily in religious contexts.
Sistren, on the other hand, fell out of usage completely by the mid-16th century, being replaced entirely with sisters. I suppose one could make the argument that since it hasn’t be used for nearly 500 years, it very well could be referred to as “not a real word”. I’m in the if-it-was-ever-a-real-word-it-still-is camp.
Interestingly enough, some feminist circles are trying to resurrect sistren, but they’ve had little success.