Oh dear OP.
I came onto this thread all ready to offer you a
but this does sound really quite mean.
I agree that you should ask your DB and his fiancée whether what you have heard (about it being tit for tat) is true. If it's not true, I don't see why they couldn't find the space for one more bridesmaid. If it is true, I think you would be within your rights not to go.
It's a lot to ask people to cough up £2.5k to attend a destination wedding when they have other financial pressures, especially if they have kids, and spending so much money to go to a wedding (and probably having to make it your main holiday) when your daughter has been deliberately excluded from being a bridesmaid just leaves a nasty taste in the mouth. It would be a bit different if she was going to be a bridesmaid and you were all going to have a fabulous family holiday, but neither you nor your daughter feel like going now.
How long has your DB been with his fiancée?
For what it's worth, the expensive destination wedding and 8+ bridesmaids are setting off all kinds of alarm bells in my head. If they are getting married in an expensive far-flung location and there are 8+ bridesmaids, how many people are they expecting to actually go to this wedding? Will there be more bridesmaids than other female guests, or are they actually expecting 100 people to find the money to fly out there?
Your DB's fiancée sounds like a bridezilla and hugely entitled, and your DB sounds daft for not putting his foot down. If he isn't willing to speak up about his niece being excluded as a bridesmaid, I'm willing to bet there are a whole load of other decisions he hasn't had any input into either.
I bet they end up divorcing within 5 years.