I think there has been some poor communication going on here. I think this kind of hit a nerve with me as I rely on my mother (and father) to look after my kids for me when I go to work and to have an odd night out (not recently with a breastfed newborn though!). It's not that I'm ungrateful but I certainly require their assistance to continue working. If they were to take holidays I would appreciate some notice on their part as I would be stuffed - and to be truthful I would appreciate that they would check with me first as they know I depend on them.
Any time I ask they always say yes. I'd hate to think they are secretly breeding contempt and other family who are not close by perceive that I'm taking advantage of them. I'd be horrified and angry to know if that they had been complaining to other family. If I was unaware of changes in their financial circumstances I would be miffed if they all of a sudden asked for money too.
So I strongly believe if your inlaws are unhappy with how they are being treated then they need to learn to speak up for themselves. Any interference can escalate the situation and turn it very ugly.
Eta: in no way am I condoning your BIL/SIL's behaviour if that was has happened with raised fists etc. But just be mindful you didn't witness it yourselves so it could of been an exaggeration and are only getting one side.