Well she hasn't wronged you from anything you have said, except for simply not upholding her oath to help ensure you walked the straight and narrow. I would take the oportunity to try and bridge the gap, or at least let her know that you hold no ill feelings for her.
Sometimes as adults we let our relationships with each other get in the way of our relationships with our kids and god-kids. My son (8) just went through something similar. We, through some very unfortunate circumstances have had to move a couple of times in the last couple of years and he has left behind one very good friend. He questioned why he was not allowed to visit that friend anymore and sadly it is because the childs mother holds some animosity towards my wife, so both kids have to now miss out

. That is a tough pill for an eight year old kid to swallow.
She may for whatever reason not wish to reconcile with you because of what happened between her and your mom. We adults can be SO petty..... Anyway it sounds like you are looking for some closure, go for it with
NO expectations, but lots of forgiveness and an open mind and heart!
God bless ya