I like the word brethren. Its specialized use is to denote members of a solemn or sacred brotherhood, sometimes including women too. Nobody would now say, I have three sisters and two brethren, unless he was telling a joke; hes a member of an order of priests, and there are three nuns next door. Brothers will do. But sometimes the older word is more powerful: Brethren, let us now consider the matter before us.
How did it get to be brethren, anyhow? The Old English word was brothor. It formed its plural in an old fashioned way: umlaut, a vowel-turn. The plural was brether. That happened because the ancient plural added a y- sound: * brothrj, the j pronounced like a hard y. Make that sound: say the old word ye. Say it loud. Your tongue is near the roof of your mouth, in front. The yeee sound influenced the vowel before it. (The sounds we pronounce alter nearby sounds all the time, doncha know?) It turned the back vowel o into the corresponding front vowel e. German still forms plurals in this way. German Bruder = brother; but Brueder (spelled in German with two dots over the u, to signify that you are going to move that vowel up front: make as if to pronounce ooo but instead try to say eee, and youll have what the French spell as u, the Anglo Saxon monks spelled as y, and the Germans spell as u with two dots over it, the umlaut sign).
So how come we dont say, My brether? Well, after a while, after one particular way of forming plurals took over all the rest, it just didnt sound plural. So English speakers tacked on another old fashioned plural after the old fashioned plural. One ox, two oxen; one brother, two brethren. The same thing happened to the old plural in r, in the word childer (compare with German Kinder ). We doubled the plural there too: children. The word brethren, then, is a little like brotherses —-a little. Gollum would like that.