this is just crazy. any family rift will be of their doing - you don't have to justify yourself at all.
maybe whenever it comes up just enthusiastically say how much you're looking forward to it, how you've booked your tickets and can't wait. ignore any mentions of money, just change the subject and keep a smile on your face!! i know it's easier said than done, but hopefully they will eventually get the message and just drop the whole thing!!
We spent about £4000 on our wedding - and that included a free bar for everyone at the evening reception!! it was a great party and many people said they enjoyed it far more than many big formal weddings they have been to (including my BIL's posh wedding do which cost about £20k )