[seishin]

i agree with everyones opinion in here but maybe you could try to talk with them to be together as a whole.what i mean is that you could try to get each one to understand that you will always be there for them(you know give them security that they can always come to you no matter what happens especially now that they are teens) after that maybe eventually they will come to an understanding and they will stop making each others life a living hell.

i mean sometimes one thing leads to the other you give them love and care (through many different ways) and that could push the negativity away

i hope everything gets better with your family good luck^^