I still think if there's a history of the SIL not liking the OP, and saying her brother shouldn't be with OP, then for her to invite just him (when being able to pay for all of them) and him accepting says to the SIL the OP's DH is fine with her attitude towards his wife. It's not about the holiday, it's about allowing someone else to belittle your family unit because they don't want to deal with the rest of them.
I don't have a problem with my DH having time away, this year it's 2 weekends and a week, and while he'd be fine with me doing the same, at the moment my friends i'm close enough too to want to go away with are either unable to for financial or practical reasons (we seem to be having a mini baby boom amongst my close friends in 2012). If this was a 'boys holiday' or even a sister who was supportive of the marriage and really could only afford to pay for one ticket, then it would be ok, but this is just her using money to get her own way. I can't stand it when people throw their money and make it look like they are being generous, when actually they are being selfish (not wanting to deal with the fact that her brother has a wife and 2 DCs who he is with even though she told him not too).
OP - your SIL can afford the ticket, I would assume she'll take at least a couple of days off to spend with her DB when he flies all that way, so she has holiday entitlement, I'd be tempted to say if this is really just about seeing his sister, she can come to you.
But then I don't get how he can get on with a sibling who doesn't respect his wife. (This of course is assuming you've not done anything terrible, beyond 'being the most important woman in his life'.)