You need a chat/message with family. Something like as you know I have taken decision to reduce hours. You could invent reason (taken on X), say you were stressed with all that was going on but that you are not prepared to justify. The only issue with an invented scenario is they are more likely to pick holes in it.
That you made this decision for you and immediate family (eg kids and husband). That sorry to be blunt but it was not so there would be a go to person for childcare.
That while you cannot speak for parents that you think it would be unfair to use them for childcare or use them as a way to make you feel guilty. After all dad is high risk.
Ask them not to rely on you for childcare as there is a strong chance you will say no.
I would suggest you have above kind of conversation now and not leave it until there is an ‘emergency’.
Your sibling made the decision to turn down alternative childcare . It is their right and they don’t need to discuss this with you first if you are not involved. However, if they are factoring you in they should have said we are thinking that us as parents with your help can make it work. I am assuming they did not.
There is helping family out and then there is being used as a doormat. I am assuming that there are financial/career implications to reduced hours. So you are making a sacrifice. It sounds like sibling did not want to sacrifice extra money for additional child care which is why it is up to them to juggle and sacrifice their time. They cannot have it both ways. Expecting you to provide the childcare.
You do not need to justify free time. It does not sound like you have it but if you did it would be because of decision/sacrifice made. If sibling starts quizzing you on time, quiz them on money they spend. Something like if you did not spend money on x you could probably spend it on childcare. They will probably say it is not for you to quiz me on what you see as perceived luxuries. So my answer would be but it is ok for you to ask me to justify my time/decisions but not for me to do the same?
The next four weeks childcare for niece/nephew is not your problem. Unless you are happy to look after them (sounds like not) make it clear you will not be looking after them.
Things between you and your OH sound unbalanced but that is a separate issue that you may or may not want to look at.