I don't know if this helps provide a different perspective.
My sister had a best friend growing up at school. She, the best friend, became part of our family - we were all in and out of each other's houses and as kids we all played together, but my sister and this girl were best friends.
As we grew up, we all had babies at the same time. My sister and the rest of my family moved away when our babies were small. Mobile phones were a thing but more of a luxury item and none of us had the money for the internet.
Her friend and I started to hang out more and more. We were teen parents, broke and we were honestly a true support for each other. Childcare, company, food when we needed sometimes. My sister, who had moved 200 miles away, went off the rails a bit. Drink, different men etc. I always was there for her and talked to her and visited as often as I could, but their friendship suffered. They coped differently with being teen parents (my sister didn't cope frankly) and this, combined with different family events, the lack of being able to afford to talk regularly on the phone/by text or the lack of willingness on both sides to write to each other meant they grew distant and in the end, stopped speaking entirely. They are both very different people at any rate.
Our friendship grew though and we became 'best friends', when I moved away, I still made sure to go to my hometown regularly and 20 years on, we are still close. This drives my sister crazy. She hates it. She rants and spits at anyone who will listen when she has too much to drink about how I stole her best friend. I am careful about what I post on social media because it can set her off.
I am caught. I love my sister and I love my friend. I am unwilling to give up this friendship.
My friend is pretty pragmatic, just sees the issue being that they grew apart like a lot of friends do and sees her as a little batty these days. She has no gripe but has no desire to reach out and try and be the friends they were as youngsters.
It's hard but there was honestly no deception or deliberate act to omit her. It's just how the cookie has crumbles.