What I DO want...is at the very least, a relationship with her, a chance to get to know her and vice versa, an opportunity to be her 'support' through this. Instead, I fear the shunning will resume and I'll go back to being shut out of her life again and forever
There's a saying...
"It's easier to put on a pair of slippers than to try and carpet the whole world".
In other words, do whatever you can within your own power and within your own circumstances rather worry about what might happen or try to control what others may do. All you can do is take things as they come, behave honorably and let time take care of the rest.
I grew up for the most part in the U.S. with all of my relatives living in Europe. As a kid, I only saw them a handful of times for a short period of time but the impact those short visits had on me was quite remarkable. A few day to a kid seems like a long time. Later in life we were able to pick up right where we left off as if no time had passed at all. There's some kind of chemistry that takes place between family members that can't be explained. I think your Niece will probably feel the same way when she's older. She will remember fondly the times you spent together and will see the subtle resemblances you have to her mother. She will be old enough to realize the situation and will see you for who you are.