I may be in the minority, but I do not really see any intentional wrong that has been done. I have had my share of SIL issues, but they are usually issues because they are truly meant to cause hurt, give a dig, compete, etc.
If I read your question right,it really does seem like your SIL's mistake was an unintentional, forgetful goof. And to be honest, if your SIL is like me and most moms who NEVER get out much, a reunion with a large group that hasn't reconnected for 10 years sounds like a pretty rare opportunity that may not happen again. I'm really betting it's not "trumping" seeing your kids and that your kids truly are important. These may be people she keeps/kept in touch with over the years by phone, email, facebook, etc, but perhaps just never sees tham because of life being too busy for everyone to finally get the same date free. These are all just assumptions I'm making...or maybe just possibly another point of view to try to see it from.
I don't know, my husband and I haven't had a kid-free weekend together in over 10 years, but know our day is coming and we're ok with it :). Part of that reason is because there was never a great time for us to ask anyone to watch them (they're less than a year and a half apart and so, like yours, a bit of work when they were that young ;). My husband actually won 3 different trips we turned down because we couldn't burden anyone for a whole week with two toddlers (Caymen Islands, Hawaii, South Africa---I know, some would think we were crazy but we couldn't do that to our older parents and really didn't have the heart to impose on friends/family for a whole 7 days when they had their own kids/chaotic life).
Anyhow, I think it is great that you two are getting away, but is it possible to reschedule your trip for the following week (or next?) when they will both be home? I agree with you that 4 kids ages 4 and under is a lot for one man and even one couple. And I know it is really irritating (legitamately) that you planned ahead and they forgot/screwed up...but was it was kind of generous for them to offer/agree to that in the first place? I'm just wondering if anyone is going to enjoy themselves under the circumstances (you are going to be worried/feel guilty about your brother being overwhelmed, resentful toward your SIL, your brother is going to be stressed! and your SIL is going to feel your resentment....none of this is good for the future health of your relationship with them which ---trust me---you would rather have "ok").
So I'm just thinking that if there is a way for you to avoid in-law drama (even if you are not the one that screwed up), you will be happier in the long run. So IF it is possible to just transfer your reservations to a following weekend when both your brother/SIL can be there, I would do that (and even still be grateful they are taking your kids). You will be taking the high road (trust me that is the one you want to take) and that will be somehting you will feel good about which will make for a really great time/experience when you two finally get away.
Just my opinion....