Nobody has classed it as "an emergency", Triggles, so you can get over your "shock".
The phrase used was "emergency childcare", which normally means a sudden need for childcare that can't be covered by whatever arrangement is usually in place, i.e. the situation described
Phrases I have used to describe the problem are "pickle" and "predicament".
But then, I don't need the people I love to be in a life or death situation to put myself out to help them.
"And as another poster stated, she managed to make other arrangements, so obviously the MIL was not her only option, so why is she getting so upset over it?"
I would guess that the OP thinks the arrangement made is a lot more problematic for her sister than it would have been for her MIL.
The fact that the OP has people who love her enough to out themselves out to that extent to help her doesn't absolve a selfish woman of her refusal to help her son out on a very important day for him because she basically couldn't be arsed.
Unless you do job interviews every day, this was not a run of the mill situation.
Taking unpaid leave to look after sick children has a massively deleterious effect on people's careers.
So on an already stressful day for the family, a child can't go to nursery and neither parent can step in without potentially serious consequences.
You could help, but you'd rather have a pedicure.
That makes you kind of a dick.