This.
I was in a similar situation.
My late husband left his first wife when he discovered that she'd slept in the same room as a colleague on a work outing. He'd long had his suspicions.
She claimed that nothing had happened, though she wanted it to. Her emotional affair - if that's what you call it - had gone on for some time. Their children were adults and had long since left the family home.
The daughter's loyalty was always to her mother. Part of that may be because the daughter was the OW involved in the break-up of her partner's marriage to his wife.
There was an age gap between my husband and me. There was the same age gap between the daughter and her partner. (There was also an age gap between the ex and her affair partner - but she was only about 7 yrs older than him, I believe.)
When the grandchild was born [6 years after we married], the daughter announced that she thought that I was 'too young' to be 'gran'. (Thinking about it, she was in a similar situation with her partner's kids and grandkids.)
My husband suggested that I could be an honorary aunty. That was shot down. "She has plenty of aunties. She's got her Aunty [the son's partner] and her Aunty [daughter's best friend]."
So, after that I just held my tongue. We later found out that the ex's Affair Partner was 'Grandad', much to my husband's dismay. When the AP died, a bid was made to have the next partner as 'Grandad' as well.
When he died, we then heard that the ex had a 'friend'. My husband's wry comment was, 'Aye...That'll be another Grandad.'
Now that my husband's gone, I'm no longer in contact with them. I finally snapped a couple of months after the funeral (which they didn't attend, citing the pandemic). But that's a whole 'nother story...
I do feel sad about it at times, but it can't be helped.