FWIW I'm here for you.

I can see why you were so upset with your sister now, given the past issues with her. I'm so sorry this is happening. I hope you can resolve with her - it may take time.
I think it's possible Una was trying to take the burden from you by saying it's not your responsibility to get involved with dealing with your nephew's abuse. I don't know, but that's how I heard it.
At any rate I hope you are okay and will call you later.