I also feel your reaching out to your nieces may be the most productive and rewarding route. Prove their mother wrong about you.
It's truly best to try to resolve differences pronto, especially in a family, before ppl get carried away with life-long grudges.
I have a very, very difficult sister. She is very divisive within the family. I would never give up on my niece. I remain open to my sister as long as she is not abusive in the moment. I try to not let the past predict my present or my future. I refuse to empower these wrongs.
Set the example. We all need love and forgiveness. Thus, it's helpful to be forgiving.

WC