When do you walk away from a family relationship?

For the last 12 years, I have had a volatile relationship with my niece. I have apologized for hurting her feelings, and have made many concessions to repair the relationship. As we were both at fault for the misunderstanding, I am the only one to apologize. I solely invest the energy into trying to repair and maintain the relationship (inviting to family events, sending holiday cards, gifts to her children, etc), in time she comes around and things appear to be on track, although one-sided. She never includes us in her family life, invites us over, etc.

Then I will say something that “triggers” her and we are back to the initial conflict. She will say “well… you did X, or didn’t do Y”...and I ask her how long will I be blamed for something that happened 12 years ago, and that I’ve apologized for?

She has 2 young children who I barely know and is expecting her third child. I’d like to be involved in their lives, although limited, but am realizing my niece might not ever forgive me, and that’s ok. If so, I just need to move on and stop trying.

Any advice?