Resolved questions
Do you you use "brethren" instead "brothers?

Actually, those words have two different meanings. A brother can be a number of things, but primarily, we use it to designate siblings (i.e. two boys born of the same mother, father, etc.). "Brethren" has a specific religious meaning- usually brethren are from the same sect or religious order.

Now, "brother" can also have a religious meaning, but it has many more uses than the word "brethren."