She's 12, which means that she's absolutely a whisker off having full autonomy regarding how much time she spends with her dad.
She doesn't actually have to like OP, want to spend time with her, or take kindly to being called a 'little madam' or adversely compared to OP's ladylike daughters.
Nor does OP really need to attend her boyfriend's niece's overseas wedding. If the relationship has legs, that'll keep.
I say this kindly after being in an unsuccessful relationship with a perfectly pleasant bloke whose teenage dd decided to loathe me. Never mind that we'd been introduced by her mum, who was a mate of mine & thought I'd hit it off with her ex.
We did, & he was great, but he was absolutely hellbent on his dd liking me. She didn't like me. I wasn't her biggest fan either - she said some thoroughly nasty & uncalled for things about me to her dad & anyone else who'd listen - but she was a teenager, I was an adult.
So I told the nice bloke that I'd be quite happy to carry on dating him, but obviously there was no room for me in wider family events because this was distressing his kid, who needed to come first.
He was insistent on forcing a relationship that simply wasn't going to happen, so I wished him well & walked away. I recommend it.
We're still vaguely in touch, so I know, his dd hates his latest girlfriend, & he STILL can't see that he could just...go out with whoever he likes...without insisting that a) every woman he dates needs to pass the dd test & b) his dd has to welcome every new gf her dad dates as part of the 'family'.
To me, it tends to smack quite a lot of 'why can't these various females just orbit ME' tbh.