Who will look after MIL after this avoidable cock up ?

AIBU to now approach SIL, voice my concerns and ask her to come up with a care plan that doesn’t rely on goodwill from friends and neighbours

YABU
Stay out of it otherwise it will massively backfire on you. It is highly emotive and if something happens to your MIL you will be blamed. Don’t get between your DH and his mother and DH and his sister.
Offer advice but don’t push it.
Leave it your DH and SIL to thrash if out.

ETA - I do hope you and your DH get to enjoy your holiday.

Surely this is SiL’s problem to fix as she booked second? Don’t do anything.

@Souredgrapes @SaladSandwichesForTea but this was part of the package of getting married pretty sure that becoming a carer for mil was not included in the marriage vows when @Souredgrapes got married!! how can you say that? the sil, on the other hand, has been quite selfish by booking a holiday on the same date as dh and OP! is sil going to still be accepting text and calls when she is away on hols or will it just be your dh being guilted? sorry but we have been there with my mil! only ones who stayed local and we were left to do it all!! one bil even suggested to my dh that he goes along every evening to play cards with her! he saw her every single day, got the emergency calls and he would rush over to discover she had mislaid her watch! all sorts of rubbish like that!!

DH needs to face some facts here. He's wrong to call you selfish when you're just being realistic.

I can't think of anywhere that would do respite care for £70 a week.....

Presumably your MIL gets Attendance Allowance? If not, sounds like she qualifies. This will give her the money to pay for respite care while you, your DH SIL take a holiday.

Going forwards you need to discuss how you, DH & SIL are going to care for MIL as her care needs increase.

Why do you think SIL is a tit? You don't have enough information to make that judgement definitively.

SIL sounds like she is working just as hard at looking after their mother as her brother is (OP's DH). She might be reaching the end of her tether and need to force a discussion about a care home or carers if her brother hasn't been open to that. This is one way to do that if he otherwise isn't listening.

It's not clear what your DH's position is. Your position is clear and your SIL's position is clear - you're both going on holiday and don't intend to provide any care for your MIL during that time (as is your right, nobody has to care). What is your DH's thinking though, that he can answer a few texts and otherwise she'll be fine left on her own? I mean it's a valid position maybe although it leaves him exposed to having to fly home early if he chooses to do so.

What's the thinking behind him not wanting to get a professional carer in? That's what you need to get to the bottom of. On the face of it it sounds like there's no reason not to but maybe he thinks she's safer on her own? Or something more sinister like he hopes she might have an "accident" and there will be nobody around?

In any event I think you are wrong to blame the SIL because she's no more obliged to change her plans or provide care than you are. When children decline to care for their parents it's usually a direct result of the kind of upbringing they were given - you reap what you sow and all that.

How did you find this PA ? I need someone like this for my parents.

I don't think she is a tit. I think she is at the end of her rope and feels the only thing to do is to force her brother to pay attention.

oh that's a tough one OP.

I would say: stay out of it except at the most superficial level. your DH and SIL now need to arrange care for MIL for while they are away. Frankly? she needs to be in a home, and this might help push them into making the decision.

The messages are a lot, but that is between DH and his mother, it doesn't seem to bother him.

But you also need to tell him that you will not be leaving your holiday early for any reason, and that he needs to think long and hard about what circumstances will provoke him to leave. Is SIL likely to cut her holiday short for any reason? He needs to think about why that might be.

But as far as you're concerned? carry on as usual. Support your DH to make the decisions that you all know need to be made.

It’s pretty straight forward. If her two main carers can’t care for her she needs someone else to. End of.

As you can image these conversations are tense as I’m coming across badly in his opinion .

In addition to what others are saying, don’t allow him to mischaracterise you. You are right to be concerned and there is nothing wrong with you wanting to have a break or wanting your husband to be present with you on that break. He doesn’t need to be writing a nasty narrative that you’re being selfish when it sounds like you’ve been quite understanding and supportive. He’s taking his feelings about his mother’s decline out on you. You aren’t his emotional punching bag, so don’t let him get away with that.

Your mil needs a lot more support now whether it be carers popping in twice a day, a live in carer, respite or moved to a permanent supported living accommodation.

She needs to go into respite care - tbh it sounds like she will be needing full time care before long anyway so it could be a good practice run.
SIL is a tit.

Agree with previous posters who have suggested that your DH and SIL could use this as an opportunity to ramp up the care being provided for your MIL (provided she has funds/can access this through SS).

At the least it feels as though a carers' assessment should be sought by DH and SIL.

But ultimately it is their issue to solve; all you can do is draw your own boundaries, keep calmly making the case for MIL to have more support, in order to reduce the 'single point of failure' risk and make DH and SIL's support of her more sustainable.

My parent has responded well to having a 'PA' (not a carer, but a very very versatile support worker) who comes in twice a week, helps with admin and organisation, paperwork, supports shopping and meals on wheels deliveries, runs errands, would supervise physio (if parent would let her!), and generally troubleshoots. My sibling and I have made the case that this greatly reduces the pressure on me, as I couldn't keep up with my parent's needs, but now when I visit it's more like 'fun' than 'help'. I don't know whether that feels like a model that your MIL, DH and SIL could find easier to accept?

No. SIL and OP's DH are both run ragged trying to care for their mother and both probably need the break.

Far better to ask to acknowledge the very trying situation and just ask to clarify what the plan for respite care is for MIL because of concerns over her safety if left alone for that length of time. Maybe also suggesting that the issue of ongoing care should also now be discussed because the status quo will probably soon become unsustainable as MIL's care needs become more and more complex.

It's best not to inflame the situation much further. It's shitty all round as things are.

Well texts & call are going to spoil the holiday as there is nothing you can do & he'll be miserable!

He & his sister need to look into respite care with a view to permanent care I would think.

Is the current situation really best for his mum?

Yet more agreeing with Sil is doing this to force the issue and rightfully so.

"DH isn’t listening to my concerns as he thinks I’m being driven by the selfish thought that Mils endless texting and calls during our holiday are going to spoil it .

It's time for your husband to face the fact that his mom needs more care than he and Sil can provide. Why is he resistant? Is he trying to save money for more inheritance?

Personally, I would stop providing care full stop. Let your H and his sister work it all out.

The answer is respite care in a care home, and hopefully you can get it. I remember my FIL going for respite care when he was very disabled and his wife just needed a break. Your DH and his sister will need to get their finger out and organise it though.

I am inclined to agree with those suggesting that this might be a situation that has been engineered by your SIL in order to force the issue of proper care for their mother. Has either of them (particularly your DH) been refusing to have that conversation?

That's the point i keep coming back to.