Firesprite, you have a very good point. She hasn't seen her grandparents much at all since they broke their first promise. This year, apart from yesterday, she's seen them once, but I was there too and I think they curb what they say if I'm there. The only reason she went yesterday was because they'd been nagging at her dad that they don't see her enough (erm, they should be able to figure out why that is!) so she went along with him because she didn't want him to have to face anymore nagging. Plus, the last time my husband spoke with them on they phone, his mother told him they hadn't seen SD's mum and she said she was cross with SD's mum because she'd text SD to say she'd been out with them. She said 'what does she think she's playing at? She's playing mind games with my granddaughter'. So, naively we thought that maybe, just maybe, it was sinking in. Sadly not. We have made a decision today that SD will not be seeing her grandparents again - they are not going to change, ever probably, and we simply cannot put her through that kind of abuse again. I'm half tempted to tell my MIL that she won't be seeing them again, and why, but I'm not sure it's a good idea.
We are still reeling from what they said, seriously cannot believe that they would say this to anyone, let alone their granddaughter, who as you rightly point out, self harms. It beggars belief, it really does. So, there aren't any pro's to continuing contact, just a whole heap of cons. As we say in this country, my in-laws have 'cooked their bacon' and enough is most definitely enough.