Wingingit - from the outset I thought YWNBU, because similar happened to friends of mine. They booked their wedding, and close friends from the same university group then decided to have theirs a month before. My friend was very upset about this, because it meant that some mutual friends had to choose which wedding to take time off work for, to attend. And most of them chose the earlier one - meaning my friend had several of his university friends missing from his own wedding.
So when it's family, they may also decide it's not worth fronting up for both weddings, and choose which one to attend - and if they pay out for your BIL's "destination wedding" (oh how I hate those!) they may not be able to come to yours. So yes, she HAS effectively "stolen your thunder" by potentially stealing some of your guests!
Reading your updates - your MIL is barking. She clearly values her other son and his fiancé more highly than you, your DH or your DD. She needs to be reined back and realise that this is NOT about her, AT ALL, but about you and your DH's need to prioritise your own life when it comes to splashing money around. You've saved and budgeted for your own wedding - most people can't just come up with an extra £2k at the drop of a hat anyway, especially not with a huge expense like a wedding going on - and it's really unreasonable to expect you to.
I suppose this isn't another one of SIL's "jokes", is it? Or has she maybe done it deliberately so you can't go? She sounds like she doesn't like you very much and certainly enjoys winding you up.
Well - I'm glad your DH is on your side and I hope this all calms down, but I still wouldn't be going to their wedding.