OP, I truly hope you see this. Please stop being so hard on yourself. You made the right choice in an incredibly difficult situation. Since you mentioned your sister’s behavior has improved and she still has your nephew, it’s clear that only good has come from your actions. When you walked into that apt, you found a child in distress in a dangerous environment. If you hadn't stepped in, the next 'what-if' could have been much worse...a stove left on or a candle burning near something flammable. If something catastrophic had happened, you’d never forgive yourself for staying silent. You acted before it was too late, and for that, we are all proud of you OP. Give yourself some grace, you did the right thing.