Who will look after MIL after this avoidable cock up ?

Id keep out of this
Your dh and sil need to decide what to do.

It sounds like she needs more care ( possibly residential or a live in carer ) anyway, especially with the dementia
It seems like SIL is doing the bulk of the support with the comparison between her workload and your dhs. I very much doubt they are equal.
This is hard on one person and tbh I’d be mortified if my kids had to wash me ( but that’s me, maybe ? )

So I’d say dh and sil need to arrange a carer. Preferably ones on rotation so there’s someone there all the time as you said MIL has the money for this.

I don’t see why SIL should change her plans no matter how many holidays she’s had.

Surely the DM pays for her own care?

Mil needs to go into respite care and any costs are split between dh and sil

She has the option of respite care or paying for care calls each day, there isn’t another option. SiL going away the same time as you may be a blessing as it’s forces your DH and her to sort out some additional care for their mother.

these were my first thoughts too, at least about professional carers.

i'm Not suggesting there's any bad blood between them.

But having navigated this hell scape for some years and still in it, it's impossible to get the Parents to agree to care sometimes!

So if that's how she's feeling, she may want to see how a crisis unfolds with herself and her brother abroad and unable to help.

I don't think I would go abroad unless my mum was in a care home because the risk of having to fly back is just too high. (I'm not too fussed about holidays for other reasons though).

So I understand you can't bear to deal with constant messages on holiday, but something else should be put in place so that the messages aren't received.

There is nothing he can do from abroad anyway.

it strikes me as a very extreme thing to do and very unfair. So my first thought was that she wants to get her mother to accept alternative care and had to take this rather drastic step for it to happen.

This.
stay out of it - let them get on with it. Don’t fly back and don’t get involved.

That's so hard for everyone, but I'm afraid it will come to a point of having to grow more backbone (not meaning to sound horrid there) and become very firm with her.

This situation will soon become totally untenable. If she won't consider residential care then she might have to have live-in carers, which she will have to pay for.

It sounds as though the property she lives in is also now unsuitable with the bathroom being upstairs. I think she will have to be made to understand that she is at or close to the limit of what can be managed in her current house. That's nobody's fault, just the way it is. She wouldn't be safe on stairs.

I'm very relieved that my own mobility impaired elderly mother lives in a bungalow and is willing to have and pay for carers. Those are the only reasons she is still there. It wouldn't be possible otherwise.

SIL is an arsehole.
MIL is taking over your life and another solution needs to be found such as a care home.

MIL will not entertain going into such a facility again.

Unfortunately she might have to when 24hr care is needed.

I kid you not , SS deemed it ok for MIL to stand at her kitchen sink and wash .

Oh yes, we were told this with my Dad.

Bed downstairs, wash at sink, use a commode.

He used a walking stick by this time so balance not great as it was!

Right. If she won't tolerate respite, then an overnight carer needs to be bought in while you're all away.

I'm not surprised about SS. In Fife, they'll happily tell families that someone with dementia can have their bed and a commode in the living room: "Just put a stair gate across the bottom of the stairs."

You either need professional carers for MIL or to place her in a respite facility for the duration fo the holiday plus a day at either side of the holiday.

(My late mother had dementia. Even at the stage that you MIL's at, you need something in place.)

Mum in law needs respite care, use thus as time for her to get used to residential care and build up a relationship with staff, in case she eventually needs permanent placement

I read it that SIL has sorted care out for the time that she is away & Op doesn't think it is good enough.

Hang on though, I can see how SIL could be painted as the bad guy here but actually my understanding of this is that DH and SIL both share care yet DH (and Op) chose to book a holiday and then tell SIL that she would be doing 💯 of the care for those ten days.
wouldn’t it have been better to discuss first, check SIL was happy to take on DH caring duties too and then book?

I wonder if she’s booked a holiday the same week because dumping duties on her is a bit of a habit?

anyway, apart from that I agree with PP you need some sort of professional carers now.

Exactly! Add a 1 to the start of £70 and a 0 to the end.

Well Op, I think that’s a shitty thing for your SIL to do. It may be that she has had enough and wants to force the issue.

Meantime, you could call adult social care yourself and report MIL as a vulnerable elderly woman, and explain that her family will all be away at the same time and are not able to care for her.

TBH it's your MIL who is worst off in this situation, given both her carers have decided to absent themselves at the same time. That's a cruel thing to do to her. How is she going to eat / shop / do daily things that she normally relies on you for?

It's one thing saying you can't cope any more - time to get professional carers in. It's another thing saying you're all going to swan off on holiday at the same time so, what, she has to just deal with it?

Of all three adults you're the one least in the firing line. Your DH and SIL are behaving like teenagers frankly.

I'd explain to your MIL slowly and carefully that there's been a mixup and both adult children will be on holiday the same week. So they'd like MIL please to help them out by going into respite care, NOT permanently but just to cover this period. Then they will come and collect her and bring her home on such and such a date. They are asking her to do this because they love her and want to make sure she will be OK while they are away. They may not have a good signal on holiday but they will read any messages she sends and text her once a day to say all is well.
If you can find a nice respite place, she may be more open to consider more outside care in future.

I’d also suspect SIL is doing bulk of caring especially personal care and has done this to chivvy things along.
What’s his plan? He can’t just leave her home alone for 10 days if she needs daily care.
He needs to sort carers or respite.

SIL has pulled a blinder. MIL will have a crisis and be forced to accept carers, which lets SIL and DH off the hook.

Seniles can live for 15-30 years, so this crisis needs to happen.

The first rule of compassionate care: setting yourself on fire doesn’t put someone else out.

I wondered this - whether this is actually forcing your DH to face up to the fact that this simply isn’t feasible to continue and she needs (for her sake) more care than they can give

you can get temporary care home care - I know when we went away before my Nan went into residential care she did a short term stay when we went away. It really helped to see what worked and what didnt

I took that to mean one or other of them sees her every day, rather than one of them does the vast majority.

Respite in a nursing home. This is a good idea for more than this holiday. If you can fund it do it a couple times a year. That way when the dementia deteriorates she has somewhere familiar to go, that family are happy with.
My grandmother chose one to recuperate from illness a few times and the transition when she needed it full time, was easy, as family knew she liked it there.