OP, I think you're hurting and lashing out at a convienent target, the partner of your nephew. This is perfectly understandable, but it is now hurting you, and for that I think you need to let it go. For yourself, I'd leave things be; you've already indicated they could bring them via text, and just forget about the whole thing. The kids will show up or not.
It really seems like there must be some further history there, and it would explain your nephew's explosion, too, but if there isn't, there will be now. You say the reason you're so upset with the wife is that she made a statement to her husband that she was disgusted at some behaviour and she has not responded to texts. I'm sorry, but a statement presumably in private and in confidence to her partner (and how does everyone know about this anyway? and it can't be about a single text - surely it is more about the initial argument and the fact of the continuation of the topic, not whatever form that took?) and non-action do not seem unforgiveable actions. Your OP indicated the wife was causing a rift and did not even mention that the nephew had had a big blow up and argument! The rift was caused by the nephew's initial reaction, and the switch to blaming the wife in the whole for thing is just bizarre, although in keeping in stuff I've been reading lately about emotional labour and how females are the 'holders' of such things and thus expected to handle them and get the blame if something goes wrong. In this case, even if the male partner makes a scene and big argument, and the female partner does nothing, she is now getting the blame for a family rift over precisely doing nothing. She has not chosen to sulk and stir things up, someone chose to bring her into it, and it seems by her non-response she is attempting to stay out of it.
But I think such considerations are beyond you now OP. You're spending a lot of emotional effort trying to think reasons into silence, but you really can't know what the personal circumstances of her parents are on that day, whether she could or could not get childcare, whether she is simply trying to stay out of the argument or is spitefully ignoring texts, or anything really. It's taking up too much of yourspace and you've now taken enough steps you can tell yourself you've had your say, and try not to speculate about what is going on in someone else's head.