Help me avoid a falling out with my sister in law

What? In general I’d agree, but these two are a couple insisting that his parents are there to serve them, at great personal expense.
You can’t have it both ways.

I would stay out of this. My usual policy is that each person in the family is responsible for their own relationship with each other person. When my brother fell out with my dad, I didn't get involved and kept my own relationship with him, my brother gets on with him fine now. I have vetoed certain events with my divorced parents which my brother attends, and everyone thinks I'm silly but I don't care.

I don't think managing other people's family relationships usually works at all, and ends up with someone feeling ganged up on. I'd be working on my own relationships with my mum and dad and be honest with them about how I felt about the brother's actions, gently suggesting how they could handle it, but not beyond that.

They know the choices, they are choosing not to action them, leave them to it.

Then why don’t they pay for childcare in a nursery like most people do and not rely on someone who they already know has poor health?

This is purely SILs and DBs fault.

They cannot blame anyone else for any of this when they are choosing such an unpredictable method of childcare.

If they lose their jobs over not having a reliable source of childcare then that’s their own fault.

Your dad's response should be dependent on his health. If the answer to the question is no then he should say so.

It's not unreasonable for your dad to feel that he's just being used. And your brother's an arse as well.

She'd get short shrift from me, but if your Dad & your brother are both manipulated by her, then you need to decide if you want to be the black sheep, & is this actually your fight ?
You can meanwhile, refuse to engage with her, see her, invite her, go to anywhere she is present, Go 100% Cold

Who on earth is this relevant ?

I think because you (quite understandably) dislike her, everything she does you now see through that light. There are a few things here that don't seem unreasonable to me.

• Taking kids when they have winter bugs that get spread: that's part and parcel of childcare. I'm not talking specific sickness / stomach bugs, just colds and sniffles that all kids get, all parents catch, all teachers/childcarers are exposed to. It was up to your parents whether or not they accepted this, because there is no way around it.
• Your brother clearly has flexibility at work to WFH when needed (though it is highly unlikely he CAN actually work while looking after his kids, adn that his work is happy with this! Indeed in many companies this is explicitly not allowed, and rightly so). Maybe his wife does not have this flexibility?

But why would she make alternative arrangements for childcare when your parents don't want her to? They wan't to look after the children, they just need to be clear if there is something come up which means they are unable to rather than leave it vague, which means she then has to ask.

You are letting your feelings for your SIL skew this entire situation, it seems everyone is happy with the childcare arrangements apart from you?

Are you having to have them this Friday?

Good then your dad just needs to tell his son which days he is not doing and SIL can leave them to sort it together?

But she IS having to ask, no one can deny that, because she isn't directly being told that they can't do it..

SIL is presumably a functioning adult who can work out for herself that "in hospital with sepsis" means no childcare that week?

tbh at this stage, OP, i would just text to your DH and his wife and say: you two are behaving like unfeeling cunts, check yourselves. Assume no childcare for the foreseeable.

At least that will be 2 fewer Christmas presents to have to buy ;)

Yes ultimately the burden is SIL and guess what if she is let down she risks loosing her income and or respect at work and she needs that money to house, raise and feed your nephews, earn a pension etc etc

The cost you place on her is emotional cost and relationship cost and relational cost within the family and you are not therefore giving help ‘without cost’ and alongside she bears the risk of her loosing her job due to unreliable childcare. She may be terrified of this and her ‘apple cart’ falling over as a result.

I am sure nephews are loved and well cared for by you and your parents - that I am certain SIL is very grateful for. Please consider walking a day in her shoes though. Honestly keeping kids well looked after and holding down a job is a monumental effort.

Compassion for all - it is difficult- please don’t make life harder for SIL.

Just a ‘she has a lot on her plate Dad it not that she doesn’t care’ will suffice for your contribution.

I'm not defending her lack of tact.

But she IS having to ask, no one can deny that, because she isn't directly being told that they can't do it.. they are leaving it open ended on the group chat, so OP will offer to do it by the sounds of it, she is the fallback plan as it sounds like she lives at home with them.

They need to just tell SIL when they can't do it so she doesn't have to then ask them.. it's not hard. It sounds like they tell her when they are going on holiday and she makes alternative arrangements, so them not being able to do it isn't a problem.. it's them not communicating clearly when they can't do it which is causing this issue!

Came here to say basically this. Regardless of where the brother is in all this, the SIL is being a shit. I would call her out on that.

I mean, I see no harm in adding a comment in the chat along the lines of, “think you’ll have to assume childcare arrangements are off what with dad being in hospital for suspected sepsis. I’m sure he’ll appreciate your thoughts are with him this week. Love and kisses etc etc”

Well it’s not your place to call her out on it.

why the fuck not? I honestly don'T get the batshit MN heirarchy of who is allowed to talk to whom. My SIL acts like a cunt to my mum and i tell her to get to fuck and not to speak to my mum like that. My bro is too pathetic to stand up for his own mother but i'm not.

See also: not speaking to feral kids bullying/hitting their own child etc etc.

All OP wants is the SIL not to say "but but what about meeeeeee" when her lovely FIL tells her he's in hospital, but to say something polite and human and with a bit of empathy before bleating on about her free childcare.

His moral compass is evidently askew anyway though, he having chosen such a very unpleasant person to love.

I don't agree with the 'performative weeping' comment btw but the dynamic looks horribly familar to me. So sorry you feel so angry and helpless, I really am. I do agree this is not your battle to have.
You falling out with SIL won't change the dynamic, just you won't see it 'live' any more. It's gutting to read, but if your DF decides to continue because it's his only way to see the kids, he will.
And your brother is letting his wife carry the blame for there being no other relationship with them unless it's transactional.
I really feel for you, and as much as possible I hope you can step back and just love your dad and let this play out. I genuinely think the only one who will suffer if you intervene is you
The drama triangle is what's happening but that doesn't mean everyone knows their part on it, or are being manipulative masterminds.

That's very ungrateful but why is it always the daughter or daughters in law who have to ask for the help, and then get the blame for any unpleasantness? Where are the sons?

This is a good example of two wrongs not making a right.
Why lie? (Anyway it’d be bound to bounce back and bite OP, perhaps with SIL snapping that henceforth OP can fill in any time her poor dad isn't able. Or that she can fund some external childcare!)
If you dishonestly downplay concern about your father’s ill health, OP, it’ll be hard to take it back and treat it sufficiently seriously, I reckon.