Hi and welcome to beyond blue.
So sad to hear what happened to your niece. I cannot imagine the impact this would be having on you.
I certainly don't anything about the family situation, except that speculation and what if type or looking for answers is normal.
Can I ask you a question - you said that you should have done something. Was this to prevent the accident? Or something else. What could you have done?
Please do not think the questions I ask need to be answered here.
You are a very caring person. Be kind to yourself - blaming ourselves is ...
Peace and comforting thoughts to you (and your niece)