Frankly I think it's pretty rude of your daughter not to invite the cousin, if she's coming such a long way.
She's coming a long way knowing that she isn't invited though. It's not as though she said "lovely if you'd like to pop in for half an hour to see me but you can't come to the wedding". Very different.
And cousins can depend a lot. When I had my wedding all cousins on both sides and their families were invited. That was 10 extra people in total.
When my cousin got married, I have no idea about the other side but that would have been an extra 15 people just on one side. They didn't invite their cousins, which was fair enough, and I totally understood why.
A cousin in Australia is unlikely to have been an inseparable support through childhood, so is very different to a cousin who lived 5 minute walk away. So again someone saying "you have to invite cousins" is coming from one side. I had a great aunt in Australia who I've met once and never met either her children or grandchildren. My mum has met her Australian cousin three times since she was 18 months (when they moved there).
Op, I think it's worth a conversation if you have that sort of relationship.
Is she:
- Coming along because they can't leave her
- Coming along but they don't expect her to come to the wedding
- Sister's humphed because she thinks the children should be there so has decided they're coming.
If it's the first two, if there is space, then I'd try and fit her in. If it's the last, I'd issue a "lovely to see her at the wedding, but it's a sit down meal so there won't be space" comment.