not to help BIL with the kids while DSis goes away for a few days?

The children are quite young (between 2 -8). However, with all the meals prepared and plenty of food, the father will be able to cope. Perhaps your sister should lower her expectations about the house being clean and the father might just slob at home with the kids in front of the TV or whatever, not planning on trips out. Let BIL experience looking after his own children on his own and gain confidence so that DS will in future feel able to have a few days away without anxiety. It might be that BIL will feel a whole new pleasure in being on his own with his children.
OP, if you don't want to take on looking after your nieces / nephews, say No. You can rest assured your BIL will cope

He works and runs his own business. He can cope with looking after some children.

I'm willing to bet when they met he wasn't sat naked in a cold dark home, all filthy and starving trying to work out how to wash, dress and feed himself, and keep the home running.... unless he has moved to her straight from his family home with mummy looking after him then he knows how to do look after people other than himself.

A man who couldn't function as a parent without me would really give me the ick, I'd just lose all respect.

I know! I might do it for dd17 - make sure there’s easy food in the house - if I was leaving her alone.

I wouldn’t do it for a grown man who is old enough to hold down a job and father children!

You said

“Any woman who decides to have 4 kids obviously knows that it will not be easy for her to take solo trips away while they're small. Where are the grandparents in all this?”

You did not make a comment exclusively to this woman”. You said any woman with 4 kids should know she cannot take solo trips and plinted to the grandparents. Why should any woman know this? Especially when fathers, such as this one, can happily take solo trips when their kids are small?

Because unless she has the IQ of an axolotl, she could obviously see what sort of father he was after the first, second and third child. Pretty sure he didn't offer to take all 3 kids alone either. So by the time you go for a 4th, it's pretty obvious that he won't be able to deal with all of them at once.

I despair ... she's enabling this incompetent man she thought was suitable dad/husband material yet clearly isn't.

I think BIL wants to have his own bit of time off while his wife is away and will dump everything on OP if she turns up.

I think that's the plan too. He'll disappear and she'll be stuck.

I would leave him to crack on. His choice to have 4 kids, plus he goes away without a 2nd thought! This will do him good.

Absolutely this.

if she doesn't already know then how stupid can she be?

He’s not a woman.

Why can’t he ask his dad/brother/friend?

Exactly why does he struggle-is he genuinely crap at parenting/tidying his own kids/house? Or does he just prefer to sit on his arse?

Honestly, I would leave him to it.

I would struggle with 4 kids under 8.

Its why I stopped at 2.

Your sis and BIL should have factored in his parenting abilities before having 4 children. Not relied upon the nearest other female.

Say yes... for your sister's benefit.. do NOTHING.. then 3 hours before he gets back, put the kids in front of a movie and order him around to actually do some work..

Or.. spend the whole weekend ordering him around.. maybe while holding a child.. BIL.. you need to make lunch.. I've got the kids.. BIL you need to tidy the lounge.. I've got the kids.. then point out to him, at every opportunity, that your SIS does this on her own.. repeatedly saying... while holding one child.. I don't know how she does it.. two of us here and it is bloody hard!!

This absolutely boils my piss.

He’s not a child. He’s a functioning adult and father of 4. He can keep them alive without bloody safety harnesses like this.

You need to say no to your sister and explain why. Her DH needs to be able to look after his own children safely. You could offer to be on standby in case their is an actual emergency, eg one of the children needs taking to A and E. But not to go round to wash up and put the children to bed.

My brother’s wife is a partner in a law firm and often talks about how lucky she is to be married to my brother and she is one of a few women she works with who can travel for work without needing to do any household / childcare prep. And that that was exceptionally rare. It’s depressing to start to realise how correct she was.

I feel embarrassed for your sister asking this and embarrassed for her selfish man-baby husband for needing it. Why on earth have four children with a man who claims to be incapable of caring for them?

It would be one thing to ask for a hand here and there but expecting you to move in (and no doubt do absolutely everything) is ridiculous.

I’m not sure who I find more reprehensible tbh. Useless fathers and husbands or the women who enable them. I cannot fathom having 4 children in a situation like this.

And the reality is he will never learn to cope if he never has to do it.

I would only consider it if during his cycling trips she had somebody move in to help her cope.

Why?

Why should she know this when the husband doesn’t and he takes solo trips all the time.

I'd say yes, because she probably won't go if her useless co-parent doesnt have help.

BUT
I would either cancel last minute after shes gone on the trip & leave him to do the lot.
Or
Turn up and direct every question back to him & actually do nothing eg What are we having for tea? I dont know, ask your dad what he's organised & what time he's going to have it ready.
What shall we do this morning? I dont know what have you planned for the weekend? Ill follow your lead.

I cannot stand unless men who dont (wont) parent their own children.

This is really sad for her. Does she want to leave him?

I would refuse to help and say exactly what you just did.

I hate the way fathers are given a pass for finding it “too hard” to manage their children by themselves.

Oh no. They are his children.

The worst part of it is that you’ll have to spend a weekend living with her useless husband. How bloody awkward and what an ask! Just because she puts up with his uselessness doesn’t mean you should.

So essentially she wants you to babysit her husband. For four days.

I'm not a fan of this at all. How pathetic.