Mancos Memories

I have attended Mancos Days, a parade and celebration of pioneer days in Mancos, Colorado, where my grandmother grew up and my mother spent many summers of her life, as well as one year during the Depression when there was no work in Telluride.
I call Mom, as I have done every two days or so on this vacation. The phone call may be the only event in her day besides going to meals and having a bath.
"Hi Mom, I went to the parade in Mancos today, for Mancos Days. I saw Racene and Martha and Gene and their families. Now I'm at Summit Lake, where Uncle Byron's fox farm used to be."
I want to share with her the happy memories of the past in Mancos, but unfortunately my words trigger a memory that she often recites, a set piece from the past at the fox farm.
"That's where Byron watched us girls one time when we were swimming in the lake. He came and watched, and we told Grandma Brown, but she just said, 'That's Byron. He just does that.'"
I've heard this story so many times.
But this time she adds a few new lines, probably just now invented: "Grandma Brown didn't care. She said, 'He just wants to play with you girls and show you what sex is so you'll be ready to be married.'"
I don't know how to respond to this. Argue, "No, Grandma Brown wouldn't say anything like that" ? Or ignore it? I don't think these are uncovered memories of childhood sexual abuse, but who knows? At any rate, it was a big experience for her as a child, and she feels that her grandmother did not respond appropriately.
The conversation ends, and I put my cell phone down and stare at the lake surrounded by pines with Mesa Verde in the distance behind it.
A beautiful place, but I feel like crying.
I don't know what core experience is at the root of what she has just told me, but I know that I can't call her and have a meaningful conversation about seeing the family at Mancos Days.
She's not there as a person to talk with, especially in phone calls. I resolve not to call again.