@TheMotherofAllDilemmas I do think that your partner's approach really makes all the difference in this.
For various reasons, my DP has fairly recently (since January) decided to become a bloody Disney dad, and it has absolutely affected how I feel about the DSC - and him, too. They're kids, so they've always been a bit annoying (my DS is too). But I now find myself in a situation where everyone has to 'be nice' to each other when they're here (translation: no one tells the DSC off, even though they're being awful) and everything has to be centred around ensuring that every second with their father is like being on bloody holiday. All the while, I have to parent DS as normal because that's what you have to do as a mother.
The DSC's behaviour (particularly DSD who is the family golden child, but actually extremely naughty and often just not very nice) has seriously deteriorated since DP decided to become a Disney dad. DSD is extremely demanding (and always gets what she wants), doesn't do anything she's asked to do (and is rude and sulky about it), is horrible to her younger brother and always tries to get him into trouble (not that either of them are getting any kind of discipline these days), and seriously annoys my DS. The last bit is particularly difficult for me, as it's very difficult to protect him when I'm not allowed to tell DSD off, however mildly (or even just to tell her to go away and leave him alone).
So I find myself dreading contact (especially on weekends) and totally distancing myself. It is not the DSC's fault that their father has decided to let them become demanding, spoilt horrors in the absence of perfectly normal expectations, boundaries and consequences. (Or that the rest of their relatives do the same when they're with them). But it does make it much harder to like them, never mind love them. It's a real shame, as I was really fond of them when they were being parented (and I was allowed to exercise any kind of authority).
But it's DP's fault. Entirely. They were nice kids when but Disney-Parenting is utterly destroying that. I do need to find a way to fix this because it is affecting how I think about DP even more than how I feel about his children. As I said, it really isn't them; it's him (it's always a DP problem, isn't it?).
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On the flip side, I'd be horrified if my ex decided to become a Disney Dad. It would be awful for DS. All children need clear boundaries and consequences from the adults in their lives. Luckily, his father would be horrified (and having none of it) if he started to behave anything like I'm now seeing in the DSC.
In fact, DS even realises that Disney parenting is not good. He's glad both his parents are willing to offer him structure and discipline, and have (totally reasonable) expectations that he will be polite and well behaved. He knows where he stands and what he needs to do. And that his parents love him.