wingingitmomma Of course YANBU. The first posters to a thread will often say you are, that seems to be Mumsnet code of operations sometimes, IMHO.
You sound very caring and sensitive about your BIL and I think it is a shame this has happened.
If you can't go, you can't go.
Clearly your SIL (if she organised the wedding) felt a bit in the shade and has upped the anti.
Would you consider your DH going without you and the baby?
"Do you think they would pull out if attending yours if you couldn't make theirs?" If your fiance could go it would at least ensure that your BIL does make it (although I would sincerely hope neither of them would pull out after pulling this).
Of course they are free to pick any date and location they like but then they cannot be surprised when people cannot come - especially if they have said it will not be this year and it will be in this country, and then to turn round and say the exact bloody opposite!
"BIL got upset that we won't be going the date has no significance it was all picked by SIL who is all about her family she doesn't care about his."
If the new bride-to-be doesn't care about her husband-to-be's family maybe your BIL will see things for what they are.
"I don't think I'm being unreasonable to be honest I get it's their choice but we would never have booked our wedding so close to theirs if it had been the other way around."
You don't need to justify yourself, you've planned and arranged and your SIL-to-be has jumped in and nabbed the limelight, sadly making it difficult for her finance and your finance.
Long term, just focus on your big day. 10K is a lot of money but if you have got it, spend it how you like.
Hope the day goes well.