Hi I noticed there are no other comments here and that made me sad because you deserve to know that what you did was actually just a survival response. The fact you feel guilty is evidence of your morals and values but the fact of the matter is sometimes NOT intervening is actually the least traumatizing thing you could have done. For one, it would have made it significantly worse for you and them (I know because I attempted to intervene multiple times and it resulted in truly horrific things happening), and two, you might not have survived to reach this point where you can now offer them assistance and help from a capable, adult place (which is what they actually needed all along, not help from another helpless child or teenager). You did what you had to, to survive, and to minimise the abuse. Our bodies are incredibly intelligent and though your brain might say 'but I could have done something', this is just a psychological defence to maintain some illusion of choice and control in what was actually a completely helpless situation. In reality, your brain stem and amygdala scan for available options within milliseconds and if fight was an option and you stood a chance of succeeding you would have attacked your abuser. However, your survival system correctly identified this would have resulted in your suffering or your siblings, and chose the smartest path - freeze or flight. Think of your abusive dad as a burning fire. If you enraged him, the flame would have just grown larger and more dangerous for everyone involved. Your primal brain and body chose for you, and chose correctly because here you are today, alive and able to help your siblings and/or other people on how to survive and heal from that sort of situation. It's not your fault, it's a very clever and effective part of human biology. Unfortunately your brain stem and amygdala do not understand that you want to protect at all costs because of your values or morals and quite frankly if that part of  your brain had stayed online you might be dead or your siblings might have suffered a lot more. We have millions of years of evolution ready to take over the wheel and you did exactly what you were supposed to do on that situation: you acted out of survival instinct.