@nonnie
Yes I would say never.
If any of my DDs grandparents did that to use, I'd never forgive them and also, the trust and relationship is then beyond repair. Once that trust is broken, that's it, it'll never be repaired and it leaves the child in a horrible situation.
We had this between my dad's mum and my mum.
It was awful. We would be made to go to my nan's house to see her and have to listen to her bitch about my mum and her family and all because she put on a good show and made herself out to be a perfect nan. She wasn't, she didn't care about us and my mum and dad were right to keep us form her for as long as they did. The moment I was old enough, I cut contact with her and haven't seen or heard from her since.