I'm sorry you and your wife are going through this, its no fun being harangued as the scapegoat. I'm really hoping for you that this is just a case of temporary insanity on their parts and as soon as the emotion dies down they (your parents and B/SIL) come to their senses again.
All this ridiculous nonsense over a freaking vacation. *shakes head*
Its amazing how some families can blow tiny things out of proportion, to the point of ripping apart families. I went though something similar a few years ago (not regarding a vacation, I just didn't sweep a mean, rude comment under the rug) and it resulted in about 10 nasty, name-calling, abusive emails from numerous family members, including my own parents. They totally went off the deep end, all for one little meek "hey, I didn't like that" comment. It resulted in a multi-year timeout and no contact between me and my entire family, because any time they contacted me it was filled with blame and anger -- they were actively trying to hurt me with words, not help me.
It took 3 years and a very engaged, creative, and talented therapist to bring my parents around and begin to "see" just how stupid and damaging their behavior was, and only recently have we been able to be semi-involved in each other's lives again. It has taken a LOT of work on my parent's part to begin repairing the damage. It has been very painful and sad, and upsetting to know that it all started from just a tiny, not-a-big-deal spark, but they just threw gasoline on it until it was out of control. They couldn't help themselves or even see what they were doing, it was just a stupid, horrible frenzy.
I'm desperately hoping you don't have to go through anything similar. Good luck, navigating family dynamics is difficult, and even more so when emotions are running high. I'd simply keep everyone at arms length until this dies down, and then in the future you may want to let them know how hurtful and upset this reaction made you. I would sincerely hope that they can recognize what a no-win, bad spot everyone put you in, and their shaming reaction made everything worse, and apologize and help repair the hurt they caused.