Help me avoid a falling out with my sister in law

Stay out of it, OP. This is between your father, your brother and your SIL. Your parents choose to keep the status quo. You may disagree with this, but it is an active choice on their part. It comes across that your dislike your SIL, which is fine - you don't have to fall out; just be polite whenever you meet up.

but that's on your bother.
She can'T force your brother to cut off his family just because they can't provide childcare and she is a rude mare.

That will be on him then, to keep up the relationship with his parents.

In your shoes? i would tell my brother that he reins her in, and tells her to treat his father with respect and understanding. And if she continues, you will unleash Your Words On Her and sod the consequences.

I’d say it depends on how many messages FIL has sent over the last two years pertaining to his health….

I'm going to be blunt here. Please understand that I certainly sympathise with your parents, your SIL is being horrible and selfish.
However your parents seem to have accepted this treatment from her so they can see their GC. I wouldn't- but they have made that choice.
So if you force SIL to ask how your poor father is, you will all know that nothing has really changed. She is making a moment's choice to be fake polite in order to achieve her ultimate goal of childcare. How is that any better? She won't mean it from her heart.
The only real solution is for your parents to stop childcare to show they won't be treated like that but they won't - so making SIL ostensibly be polite changes nothing in reality does it?

No woman on earth, no matter how self centred, would read a message from their in laws about health problems and not reply something like 'oh dear, so sorry to hear that. Should we rearrange Friday?' or similar.

' OK but what about childcare?', nah. I'd have to see that to believe it.

I’ve been admitted to hospital many times. Some things I personally find more stressful- for example my own children’s health. I think mothers have a very different take on life to be honest

This is irrelevant.

He should be saying to SIL to give her head a wobble, arranging proper childcare and also making it clear to your parents that they won’t lose contact with the grandkids, irrespective of what his wife has to say about it, because he will bring them to visit on his own.

No reason why he couldn’t be doing this, even if he didn’t realise she was such a nut job. You seem to be making quite a few excuses for him for some reason?

Every time though? Also, presumably when he is texting in the group chat, he is actually ok.

ive got to be honest ive actually got a FIL who is similar and the running commentary of his health problems does become a bit overwhelming sometimes. I care about him a lot actually, but I’m already keeping track of so much that I really only want to know if something needs to be done. If it’s just for info, I’m actually ok to hear of it when I see him.

If I were you, I'd ignore her comment about whether they can do child care and really go to town on the group showing concern about your DF'S health. If need be get your DP to pop in a comment too to show sympathy. Totally ignore the childcare comment from her and advise your parents to leave her hanging with a 'oh we'll have to see, I'm sure we all want Dad fit and well as soon as possible'.

Sorry OP but that’s the bare minimum. He still backs up his wife and calls your dad childish, he’s just as bad! I do think your anger should be directed at your DB for not speaking to his wife about the way she treats his family.

For what it’s worth, I think she sounds absolutely awful. I probably wouldn’t be able to hold my tongue and would call it out in the chat. But DB is equally as responsible.

I see this all the time on here, though. An OP blaming the sibling’s spouse, not the sibling, for presumably joint behaviour — I imagine it’s easier to project onto the non-blood relative, assume the blood relative has no agency, as well as a weird, gendered assumption that women are responsible for family relationships.

Otherwise, as in this case, the OP would have to acknowledge that her brother is exploiting his own parents.

My father is also elderly and has heart failure. When I ask how he is he says ‘keeping on keeping on, love. How are you?’ Because he is aware that being a mum to small kids with a full time job is also tough and doesn’t like to burden me overly. He knows I care and doesn’t need constant texts from me to reassure him of that.

Dad is elderly and is likely to have more health issues than his kids. But I see nothing in OP 's posts to indicate that no-one else feels able to share that they're unwell.

It sounds like the men of the family are used to being mollycoddled to be honest. Everyone having to be ever so thoughtful to dad as he’s been having health problems whilst presumably everyone else has been problem free for the last two years?

Again, how likely is it that she will completely withhold the children? She is relying on your DP for childcare, which indicates her own family don't have a lot of spare time or the ability - would she REALLY cut off her nose to spite her face?

Why do you keep making SIL a scapegoat? Your brother agrees with her yet you all protect him like a mother hen.

Well then, your brother is as awful as his wife. They're probably well matched.

I kind of agree with this as it sounds like falling out with the SIL would be no great loss to the OP! And it sounds like someone needs to say it.

Also, is everyone posting their problems on the group chat for people to be ever so empathetic about, or is it just your dad??