I agree. Surprised by all the opinions here. @Aussierelative please support your DD and DH. Even if there's space.
Also, does niece actually even want to come? Or is DSis stirring?
If niece has serious MH issues, I can't see anything WORSE than coming to a formal social event which (rightly so) will be structured around the bride and groom and friends.
If it was one of those big ones with 1000 guests paid for by grandparents as a family reunion it's different. But it's clearly not.
Unfamiliar country, unfamiliar people, stick to schedule, set food menu, formal dining, quiet during ceremony, she can't walk out and decompress when she wants ...
Even if you're super-successful with perfect MH most wedding guests don't engage with people they don't know.
Often people stick to their own partners/conversations and rarely include newcomers.
It won't be a relaxed socialising opportunity for niece to make new girlfriends and bond with cousin and build up a British social life (if that's what DSis thinks will happen).
If her parents are trying to care for her, surely they'd be better taking turns in a cosy AirBnB/hotel room where they can control the environment more.
If they want family bonding and chat and niece to enjoy UK then a walk and an informal pub lunch would work better.
As someone with ND/anxiety traits myself I know new events can be overwhelming (so I regulate myself by steering clear).
There's often pressure to externally normalise by turning to to big events, when most are completely socially irrelevant (unless you know the hosts well and there's going to be a future connection).
Some people can be difficult/disruptive at formal events. I have some distant relatives who couldn't make small talk, but would comment/share immediate thoughts about brides appearance/weight etc. Or have issues with menu or drinks (not communicated in advance) and then expect bride and groom to resolve.
(Looking back I think mum and one daughter were undiagnosed ND, you see daughter posting about her ADHD on social media now).
Assume this is what bride has observed in the past with her cousin, and wants to avoid? It's her day.
Anxiety and MH issues can make people dysregulated and self-focussed and unaware of how their immediate behaviour is impacting others (and I should know!).
It's not fair on DD or cousin really.